Correct one Doxygen entry, replacing NOMEM with AVERROR(ENOMEM).

Originally committed as revision 9762 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/x11grab.c

@@ -79,7 +79,7 @@ typedef struct x11_grab_s
  * @param s1 Context from avformat core
  * @param ap Parameters from avformat core
  * @return <ul>
- *          <li>ENOMEM no memory left</li>
+ *          <li>AVERROR(ENOMEM) no memory left</li>
  *          <li>AVERROR(EIO) other failure case</li>
  *          <li>0 success</li>
  *         </ul>