docs: fix comment in `put_slice()` #3120

Job	Run time
semver	18s
Check Style	6s
Verify hyper.h is up to date	21s
Test C API (FFI)	39s
Test cargo-c support (FFI)	18s
Check MSRV	2m 18s
features	35s
Test beta on ubuntu-latest	52s
Test with Miri	2m 19s
udeps	24s
Test beta on macOS-latest	1m 15s
Test stable on windows-latest	2m 11s
Test beta on windows-latest	2m 9s
Test stable on macOS-latest	1m 0s
minimal-versions	41s
Test nightly on windows-latest	2m 40s
Test stable on ubuntu-latest	56s
Test nightly on ubuntu-latest	1m 15s
Test nightly on macOS-latest	1m 53s
Check exposed types	17s
Build docs	24s
CI is green	3s
	22m 54s

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