Fix typo in shader test The only error in the test should be the non-integer index. gl_FragColor should be used correctly. BUG=angleproject:1254 TEST=angle_unittests Change-Id: I0b7a36be5485661cc03bf2c476aaacae5d3b6e76 Reviewed-on: https://chromium-review.googlesource.com/320280 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>