make fullscreen video caption text a little bit bigger

fixes: #CNVS-13792

test plan:
watch a video in fullscreen that has captions,
they should be bigger than they were

Change-Id: I11b1fbb32c50a7f59fe536becdfda9e9026613cb
Reviewed-on: https://gerrit.instructure.com/37489
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>