Fix @file in matroskadec.c, it was still matroska.c

Originally committed as revision 9376 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 7c1618a..e41013d 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -20,7 +20,7 @@
  */
 
 /**
- * @file matroska.c
+ * @file matroskadec.c
  * Matroska file demuxer
  * by Ronald Bultje <[email protected]>
  * with a little help from Moritz Bunkus <[email protected]>