Fixes StartTimer's doc with the verb 'be'
that was previously missing in 'can also used'.

Change-Id: I4b3e6103fbf62d676056d32fcce4618536b7c05c
Reviewed-on: https://go-review.googlesource.com/c/go/+/165117Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>