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


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Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>