fix: proposer_index rustdoc (#1443) #4394

Job	Run time
doctest	2m 13s
check no_std	46s
clippy	1m 6s
feature-checks	6m 57s
test ubuntu-latest stable --no-default-features	1m 46s
wasm-unknown	1m 42s
test ubuntu-latest stable	1m 42s
wasm-wasi	2m 33s
test ubuntu-latest stable --all-features	2m 35s
test ubuntu-latest nightly --no-default-features	2m 39s
cfg-check	1m 19s
test ubuntu-latest nightly	2m 40s
test ubuntu-latest nightly --all-features	2m 50s
docs	2m 0s
test ubuntu-latest 1.79 --no-default-features	58s
test ubuntu-latest 1.79	51s
test windows-latest stable --no-default-features	4m 31s
fmt	13s
test windows-latest stable	3m 50s
test windows-latest stable --all-features	4m 35s
test windows-latest nightly --no-default-features	5m 57s
test windows-latest nightly	6m 11s
test windows-latest nightly --all-features	6m 11s
test windows-latest 1.79 --no-default-features	2m 12s
test windows-latest 1.79	2m 13s
	1h 10m 30s

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