avidec: add .category

While here make .class_name consistent with other AVClass.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/avidec.c

@@ -84,10 +84,11 @@ static const AVOption options[] = {
 };
 
 static const AVClass demuxer_class = {
-    .class_name = "AVI demuxer",
+    .class_name = "avi",
     .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,
+    .category   = AV_CLASS_CATEGORY_DEMUXER,
 };