time: fix godoc links; remove redundant Sub mention
The Time.Sub function is mentioned in the previous sentence.
Change-Id: Ic79958cea4d1f9f9014f6dafd8e749c204db4f50
Reviewed-on: https://go-review.googlesource.com/c/go/+/606575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>