Semconv pages: don't warn about external links to spec

[chalin]

• Contributes to Adjust to move Semantic Conventions out of OTel spec #2721

Concretely, this eliminates the following warnings (which are no longer relevant because the pages have moved):

WARNING: link to spec page encoded as an external URL, but should be a local path, fix this upstream;
  File: /Users/chalin/git/lf/open-telemetry/opentelemetry.io/tmp/otel/specification/metrics/semantic_conventions/http-metrics.md 
  Link: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-advice
WARNING: link to spec page encoded as an external URL, but should be a local path, fix this upstream;
  File: /Users/chalin/git/lf/open-telemetry/opentelemetry.io/tmp/otel/specification/metrics/semantic_conventions/http-metrics.md 
  Link: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-advice