testing: fix missing verb in StartTimer doc

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>

src/testing/benchmark.go

@@ -104,7 +104,7 @@ type B struct {
 }
 
 // StartTimer starts timing a test. This function is called automatically
-// before a benchmark starts, but it can also used to resume timing after
+// before a benchmark starts, but it can also be used to resume timing after
 // a call to StopTimer.
 func (b *B) StartTimer() {
 	if !b.timerOn {