ffmpeg: Clarify wording of ffmpeg --> avconv deprecation message
doc: Fix syntax and logical errors in avconv stream combination example

Bug-Id: 661
CC: libav-stable@libav.org
(cherry picked from commit 775a0b04f0cf8102fe322b2ee03fe1a0633dea04)
Signed-off-by: Diego Biurrun <diego@biurrun.de>