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>

ffmpeg.c

@@ -4181,7 +4181,7 @@ static const OptionDef options[] = {
     { "f", HAS_ARG, {(void*)opt_format}, "force format", "fmt" },
     { "i", HAS_ARG, {(void*)opt_input_file}, "input file name", "filename" },
     { "y", OPT_BOOL, {(void*)&file_overwrite}, "overwrite output files" },
-    { "map", HAS_ARG | OPT_EXPERT, {(void*)opt_map}, "set input stream mapping", "file:stream[:syncfile:syncstream]" },
+    { "map", HAS_ARG | OPT_EXPERT, {(void*)opt_map}, "set input stream mapping", "file.stream[:syncfile.syncstream]" },
     { "map_meta_data", HAS_ARG | OPT_EXPERT, {(void*)opt_map_meta_data}, "DEPRECATED set meta data information of outfile from infile",
       "outfile[,metadata]:infile[,metadata]" },
     { "map_metadata", HAS_ARG | OPT_EXPERT, {(void*)opt_map_metadata}, "set metadata information of outfile from infile",