Slightly clarify bitstream filter help output.

patch by Stefano Sabatini, stefano.sabatini-lala poste it

Originally committed as revision 13494 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3754,9 +3754,9 @@ static const OptionDef options[] = {
     { "muxdelay", OPT_FLOAT | HAS_ARG | OPT_EXPERT, {(void*)&mux_max_delay}, "set the maximum demux-decode delay", "seconds" },
     { "muxpreload", OPT_FLOAT | HAS_ARG | OPT_EXPERT, {(void*)&mux_preload}, "set the initial demux-decode delay", "seconds" },
 
-    { "absf", OPT_FUNC2 | HAS_ARG | OPT_AUDIO | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream filter" },
-    { "vbsf", OPT_FUNC2 | HAS_ARG | OPT_VIDEO | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream filter" },
-    { "sbsf", OPT_FUNC2 | HAS_ARG | OPT_SUBTITLE | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream filter" },
+    { "absf", OPT_FUNC2 | HAS_ARG | OPT_AUDIO | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream_filter" },
+    { "vbsf", OPT_FUNC2 | HAS_ARG | OPT_VIDEO | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream_filter" },
+    { "sbsf", OPT_FUNC2 | HAS_ARG | OPT_SUBTITLE | OPT_EXPERT, {(void*)opt_bsf}, "", "bitstream_filter" },
 
     { "default", OPT_FUNC2 | HAS_ARG | OPT_AUDIO | OPT_VIDEO | OPT_EXPERT, {(void*)opt_default}, "generic catch all option", "" },
     { NULL, },