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

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@9762 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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>