ffmpeg: modify help text for the map option

Change "file:stream[:syncfile:syncstream]" to
"file.stream[:syncfile.syncstream]", the latter syntax is more
consistent with the stream identifier form printed by
av_dump_format(), and more readable. Both syntaxes are currently
accepted by the opt_map() code.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>