time: remove extra space in docstring

Probably went unnoticed because HTML normalizes multiple space
characters into one, unless you explicitly ask for them with  .

Change-Id: I3f97b24a111da3f0f28894f1246388018beb084e
Reviewed-on: https://go-review.googlesource.com/54570Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>