fix documentation

git-svn-id: https://ffmpegsource.googlecode.com/svn/trunk@756 dcf88d3e-5074-11de-ac45-ff9ca5716a9b
Fix direct use of av_pix_fmt_descriptors

(cherry picked from commit e9076142f0f046eef48049243e0c2be51500eae0)

From: Derek Buitenhuis <derek.buitenhuis@gmail.com>

git-svn-id: https://ffmpegsource.googlecode.com/svn/trunk@757 dcf88d3e-5074-11de-ac45-ff9ca5716a9b