Some warnings were being missed, because vet's regex that finds
formatting directives was missing the 'T' verb.

Fixes #24646.

Change-Id: I2f6f9ed19e7daf9a07175199f428a62e94799ea9
Reviewed-on: https://go-review.googlesource.com/111357
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>