ffmpeg_opt: move help text for -ab/-b:a to audio category

Since at least 4.4.3, -ab/-b:a help text was in the video section
of ffmpeg -h, but these are audio options.

Signed-off-by: Marth64 <marth64@proxyid.net>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

fftools/ffmpeg_opt.c

@@ -1637,8 +1637,6 @@ const OptionDef options[] = {
     { "force_key_frames", OPT_VIDEO | OPT_STRING | HAS_ARG | OPT_EXPERT |
                           OPT_SPEC | OPT_OUTPUT,                                 { .off = OFFSET(forced_key_frames) },
         "force key frames at specified timestamps", "timestamps" },
-    { "ab",           OPT_VIDEO | HAS_ARG | OPT_PERFILE | OPT_OUTPUT,            { .func_arg = opt_bitrate },
-        "audio bitrate (please use -b:a)", "bitrate" },
     { "b",            OPT_VIDEO | HAS_ARG | OPT_PERFILE | OPT_OUTPUT,            { .func_arg = opt_bitrate },
         "video bitrate (please use -b:v)", "bitrate" },
     { "hwaccel",          OPT_VIDEO | OPT_STRING | HAS_ARG | OPT_EXPERT |
@@ -1680,6 +1678,8 @@ const OptionDef options[] = {
     { "acodec",         OPT_AUDIO | HAS_ARG  | OPT_PERFILE |
                         OPT_INPUT | OPT_OUTPUT,                                    { .func_arg = opt_audio_codec },
         "force audio codec ('copy' to copy stream)", "codec" },
+    { "ab",             OPT_AUDIO | HAS_ARG | OPT_PERFILE | OPT_OUTPUT,            { .func_arg = opt_bitrate },
+        "audio bitrate (please use -b:a)", "bitrate" },
     { "atag",           OPT_AUDIO | HAS_ARG  | OPT_EXPERT | OPT_PERFILE |
                         OPT_OUTPUT,                                                { .func_arg = opt_old2new },
         "force audio tag/fourcc", "fourcc/tag" },