avformat/cache: rename the class name fro Cache to cache

liuqideMacBook-Pro:build liuqi$ ffmpeg --help full | grep cache
cannot find cache protocol options.

after patch:
bogon:dash liuqi$ ./ffmpeg --help full | grep cache
cache AVOptions:
can find the cache AVOptions after patch.

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/cache.c

@@ -326,7 +326,7 @@ static const AVOption options[] = {
 };
 
 static const AVClass cache_context_class = {
-    .class_name = "Cache",
+    .class_name = "cache",
     .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,