doc/examples/metadata: Constify values from av_dict_get()

Treat values returned from av_dict_get() as const, since they are
internal to AVDictionary.

Signed-off-by: Chad Fraleigh <chadf@triularity.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

doc/examples/metadata.c

@@ -34,7 +34,7 @@
 int main (int argc, char **argv)
 {
     AVFormatContext *fmt_ctx = NULL;
-    AVDictionaryEntry *tag = NULL;
+    const AVDictionaryEntry *tag = NULL;
     int ret;
 
     if (argc != 2) {