lavf/riff: Fix a comment.

FourCC aivx refers to (Sony) XAVC, not (Panasonic) AVC-Intra.
Spotted by Kieran Kunhya and Vincent Olivier.

libavformat/isom.c

@@ -167,7 +167,7 @@ const AVCodecTag ff_codec_movvideo_tags[] = {
     { AV_CODEC_ID_H264, MKTAG('a', 'i', '1', '3') }, /* AVC-Intra 100M 1080p24/30/60 */
     { AV_CODEC_ID_H264, MKTAG('a', 'i', '1', '5') }, /* AVC-Intra 100M 1080i50 */
     { AV_CODEC_ID_H264, MKTAG('a', 'i', '1', '6') }, /* AVC-Intra 100M 1080i60 */
-    { AV_CODEC_ID_H264, MKTAG('a', 'i', 'v', 'x') }, /* AVC-Intra 200M 4kp24 */
+    { AV_CODEC_ID_H264, MKTAG('a', 'i', 'v', 'x') }, /* XAVC 4:2:2 10bit */
     { AV_CODEC_ID_H264, MKTAG('A', 'V', 'i', 'n') }, /* AVC-Intra with implicit SPS/PPS */
 
     { AV_CODEC_ID_MPEG1VIDEO, MKTAG('m', '1', 'v', '1') }, /* Apple MPEG-1 Camcorder */