fix string formatting so 'test -h' does not crash

Testing Done:
PANTS_DEV=1 ./pants goal test -h

doesn't crash, unlike

./pants goal test -h

https://travis-ci.org/pantsbuild/pants/builds/41402745

Bugs closed: 805

Reviewed at https://rbcommons.com/s/twitter/r/1353/
Sync of https://github.com/pantsbuild/pants/commit/21551b078b3b0fdc8cfda6510e818461bb2a99d6