examples/muxing.c: remove doxy reference to unexisting file

doc/examples/muxing.c

@@ -24,7 +24,6 @@
  * @file
  * libavformat API example.
  *
- * @example libavformat/output-example.c
  * Output a media file in any supported libavformat format.
  * The default codecs are used.
  */