bytes: fix Cut godoc typo

bytes: fix Cut godoc typo

If sep does not appear in s, we return nil here
rather than the empty string. Presumably the docs
were copied from the  strings package implementation
and brought that along.

Change-Id: I3706c94fe04abd9e2a2c8840f7cc6116386965c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354969
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Alexander Rakoczy <alex@golang.org>