bytes: fix a typo

Change-Id: Iecbfe986da386b5c9b8c366904f659acc8f34cfc
GitHub-Last-Rev: ed6c744bbd9ff61874e8dd92f4cef01851ed1f09
GitHub-Pull-Request: golang/go#69039
Reviewed-on: https://go-review.googlesource.com/c/go/+/608015


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>