os: fix 63703.md release notes

os: fix 63703.md release notes

63703.md contains a paragraph that shouldn't be there,
remove it.

While here, fix a test error message related to the #63703
implementation.

Updates #63703.

Change-Id: I82a8b0b7dfa8f96530fb9a3a3aa971e03970f168
Reviewed-on: https://go-review.googlesource.com/c/go/+/569195
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>