decoder/ffmpeg: fix ArtistSort and AlbumArtistSort mapping

These were added 11 years ago in commit 766b9fd453, but I cannot find
any evidence in the FFmpeg repository that these names were ever
supported.  This commit adds the tags as they are currently present in
libavformat/mov.c.

src/decoder/plugins/FfmpegMetaData.cxx

@@ -32,9 +32,9 @@ extern "C" {
 
 static constexpr struct tag_table ffmpeg_tags[] = {
 	{ "year", TAG_DATE },
-	{ "author-sort", TAG_ARTIST_SORT },
 	{ "album_artist", TAG_ALBUM_ARTIST },
-	{ "album_artist-sort", TAG_ALBUM_ARTIST_SORT },
+	{ "sort_album_artist", TAG_ALBUM_ARTIST_SORT },
+	{ "sort_artist", TAG_ARTIST_SORT },
 
 	/* sentinel */
 	{ nullptr, TAG_NUM_OF_ITEM_TYPES }