bytes: fix up godoc link errors

bytes: fix up godoc link errors

[Reader.Reader] does not exist in this package.
These docs most likely meant to refer to [Reader] itself.

Change-Id: I4fa117e8541573e0be7c13751fdfc102d5f2d604
Reviewed-on: https://go-review.googlesource.com/c/go/+/582515
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>