doc/examples/muxing: Fix av_frame_make_writable usage

This patch moves the av_frame_make_writable() call from fill_yuv_image
to get_video_frame so that its argument can be the actual frame that
will be sent to the encoder.

This fixes data corruption issues in codecs that keep references on
one or several previous frames.

Signed-off-by: Sam Hocevar <sam@hocevar.net>
Reviewed-by: wm4
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>