Update main.go

Fix Album Artist Metadata. 
=albumartistname

main.go

@@ -774,12 +774,12 @@ func writeM4a(w *mp4.Writer, info *SongInfo, meta *AutoGenerated, data []byte, t
 					if len(meta.Data) > 0 {
 						album := meta.Data[0]
 
-						err = addMeta(mp4.BoxType{'a', 'A', 'R', 'T'}, meta.Data[0].Relationships.Tracks.Data[index].Attributes.ArtistName)
+						err = addMeta(mp4.BoxType{'a', 'A', 'R', 'T'}, album.Attributes.ArtistName)
 						if err != nil {
 							return err
 						}
 
-						err = addMeta(mp4.BoxType{'s', 'o', 'a', 'a'}, meta.Data[0].Relationships.Tracks.Data[index].Attributes.ArtistName)
+						err = addMeta(mp4.BoxType{'s', 'o', 'a', 'a'}, album.Attributes.ArtistName)
 						if err != nil {
 							return err
 						}