Correct comment on String::copy_into_slice #3224

Job	Run time
docs	6m 57s
build-fuzz	2m 26s
cargo-deny (advisories)	3m 11s
cargo-deny (bans licenses sources)	3m 26s
build-and-test (ubuntu-latest, x86_64-unknown-linux-gnu)	9m 18s
build-and-test (macos-latest, x86_64-apple-darwin)	25m 6s
build-and-test (macos-latest, aarch64-apple-darwin)	16m 23s
build-and-test (windows-latest, x86_64-pc-windows-msvc)	25m 43s
fmt	9s
publish-dry-run	0s
rust-analyzer-compat	10m 28s
complete	1s
	1h 43m 8s

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