demux: fix a comment

Obviously doesn't sense with this order. The git history shows that this
comment was touched multiple times, without ever fixing it. It was
originally added in 2016, where the "for" was missing. Later, the "for"
was added, but to the wrong position.

What the fuck?

demux/demux.c

@@ -2696,7 +2696,7 @@ static void update_final_metadata(demuxer_t *demuxer, struct timed_metadata *tm)
 
     struct mp_tags *dyn_tags = NULL;
 
-    // Often for useful audio-only files, which have metadata in the audio track
+    // Often useful for audio-only files, which have metadata in the audio track
     // metadata instead of the main metadata, but can also have cover art
     // metadata (which libavformat likes to treat as video streams).
     int astreams = 0;