cmd/vet: don't treat trailing % as possible formatting directive

cmd/vet: don't treat trailing % as possible formatting directive

Eliminates the following false positive:

cmd/go/go_test.go:1916: possible formatting directive in Error call

The line in question:

tg.t.Error("some coverage results are 0.0%")

Updates #11041

Change-Id: I3b7611fa3e0245714a19bd5388f21e39944f5296
Reviewed-on: https://go-review.googlesource.com/27128
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>