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>
ffmpeg: Do av_buffersink_set_frame_size() when reconfiguring the filtergraph not just when changing audio resample parameters

Fixes Ticket3562

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