[mdatagen] Improvement on README for mdatagen new codegen file

[dgoscn]

There are some outdated info about the generation of mdatagen.go on readme contrib project. Some wrong links like
/receiver/hostmetricsreceiver/internal/scraper/cpuscraper/codegen.go is present, but the file itself, doesn't exist anymore beeing the correct name: doc.go

Link to tracking Issue: open-telemetry/opentelemetry-collector#10904

[runforesight]

Foresight Summary

    
Major Impacts

TestRabbitmqIntegration ❌ failed 1 times in 1 runs (100% fail rate).

TestRabbitmqIntegration/Running_rabbitmq_3.9 ❌ failed 1 times in 1 runs (100% fail rate).

build-and-test-windows duration(4 seconds) has decreased 35 minutes 48 seconds compared to main branch avg(35 minutes 52 seconds).

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✅  check-links workflow has finished in 1 minute 20 seconds (1 minute 32 seconds less than main branch avg.) and finished at 7th Dec, 2022.

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✅  tracegen workflow has finished in 1 minute 18 seconds (2 minutes 1 second less than main branch avg.) and finished at 7th Dec, 2022.

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✅  prometheus-compliance-tests workflow has finished in 3 minutes 41 seconds (5 minutes 22 seconds less than main branch avg.) and finished at 7th Dec, 2022.

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✅  load-tests workflow has finished in 17 minutes 17 seconds and finished at 7th Dec, 2022.

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loadtest (TestIdleMode)	-     🔗	✅ 1  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceNoBackend10kSPS|TestTrace1kSPSWithAttrs)	-     🔗	✅ 8  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestTraceBallast1kSPSWithAttrs|TestTraceBallast1kSPSAddAttrs)	-     🔗	✅ 10  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestMetric10kDPS|TestMetricsFromFile)	-     🔗	✅ 6  ❌ 0  ⏭ 0    🔗	See Details
loadtest (TestMetricResourceProcessor|TestTrace10kSPS)	-     🔗	✅ 12  ❌ 0  ⏭ 0    🔗	See Details
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✅  build-and-test workflow has finished in 38 minutes (25 minutes 45 seconds less than main branch avg.) and finished at 7th Dec, 2022. There are 2 test failures.

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unittest-matrix (1.18, internal)	-     🔗	✅ 592  ❌ 0  ⏭ 0    🔗	See Details
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correctness-metrics	-     🔗	✅ 2  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, processor)	-     🔗	✅ 1465  ❌ 0  ⏭ 0    🔗	See Details
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unittest-matrix (1.19, receiver-0)	-     🔗	✅ 2533  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-0)	-     🔗	✅ 2533  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, receiver-1)	-     🔗	✅ 1845  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, other)	-     🔗	✅ 4355  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.18, exporter)	-     🔗	✅ 2416  ❌ 0  ⏭ 0    🔗	See Details
unittest-matrix (1.19, receiver-1)	-     🔗	✅ 1845  ❌ 0  ⏭ 0    🔗	See Details
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integration-tests	-     🔗	✅ 57  ❌ 2  ⏭ 0    🔗	See Details
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⭕  changelog workflow has finished in 3 seconds (5 minutes 6 seconds less than main branch avg.) and finished at 7th Dec, 2022.

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changelog	-     🔗	N/A	See Details

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⭕  build-and-test-windows workflow has finished in 4 seconds (35 minutes 48 seconds less than main branch avg.) and finished at 7th Dec, 2022.

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Job	Failed Steps	Tests
windows-unittest-matrix	-     🔗	N/A	See Details
windows-unittest	-     🔗	N/A	See Details

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