doc: Fix syntax and logical errors in avconv stream combination example

Bug-Id: 661
CC: libav-stable@libav.org
Merge commit '775a0b04f0cf8102fe322b2ee03fe1a0633dea04'

* commit '775a0b04f0cf8102fe322b2ee03fe1a0633dea04':
  doc: Fix syntax and logical errors in avconv stream combination example

Conflicts:
	doc/ffmpeg.texi

Merged-by: Michael Niedermayer <michaelni@gmx.at>