docs: use -bsf:[vas] instead of -[vas]bsf.

The latter syntax is now invalid.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/avconv.texi

@@ -799,10 +799,10 @@ Set bitstream filters for matching streams. @var{bistream_filters} is
 a comma-separated list of bitstream filters. Use the @code{-bsfs} option
 to get the list of bitstream filters.
 @example
-avconv -i h264.mp4 -c:v copy -vbsf h264_mp4toannexb -an out.h264
+avconv -i h264.mp4 -c:v copy -bsf:v h264_mp4toannexb -an out.h264
 @end example
 @example
-avconv -i file.mov -an -vn -sbsf mov2textsub -c:s copy -f rawvideo sub.txt
+avconv -i file.mov -an -vn -bsf:s mov2textsub -c:s copy -f rawvideo sub.txt
 @end example
 
 @item -tag[:@var{stream_specifier}] @var{codec_tag} (@emph{output,per-stream})

doc/bitstream_filters.texi

@@ -57,7 +57,7 @@ stream (carrying the AVI1 header ID and lacking a DHT segment) to
 produce fully qualified JPEG images.
 
 @example
-avconv -i mjpeg-movie.avi -c:v copy -vbsf mjpeg2jpeg frame_%d.jpg
+avconv -i mjpeg-movie.avi -c:v copy -bsf:v mjpeg2jpeg frame_%d.jpg
 exiftran -i -9 frame*.jpg
 avconv -i frame_%d.jpg -c:v copy rotated.avi
 @end example