ffmpeg: Fix typos in the comment for decode() ("." vs. "->")

pkt is a pointer, so it should be dereferenced with the -> operator.

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>