Correct comment on String::copy_into_slice #3220

Job	Run time
build-fuzz	2m 1s
build-and-test (ubuntu-latest, x86_64-unknown-linux-gnu)	6m 51s
build-and-test (macos-latest, x86_64-apple-darwin)	22m 9s
build-and-test (macos-latest, aarch64-apple-darwin)	17m 18s
build-and-test (windows-latest, x86_64-pc-windows-msvc)	17m 6s
docs	4m 50s
cargo-deny (advisories)	2m 37s
cargo-deny (bans licenses sources)	3m 15s
fmt	7s
publish-dry-run	0s
rust-analyzer-compat	9m 12s
complete	1s
	1h 25m 27s

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