Fix doxygen for libvlc_audio_output_set

Tests show that current docs are wrong.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit fd4a0ddc54b4571272e68d29c9ad733b9544d811)

include/vlc/libvlc_media_player.h

@@ -1094,7 +1094,7 @@ VLC_PUBLIC_API void libvlc_audio_output_list_release( libvlc_audio_output_t *p_l
  * \param p_mi media player
  * \param psz_name name of audio output,
  *               use psz_name of \see libvlc_audio_output_t
- * \return true if function succeded
+ * \return 0 if function succeded, -1 on error
  */
 VLC_PUBLIC_API int libvlc_audio_output_set( libvlc_media_player_t *p_mi,
                                             const char *psz_name );