doxy: Format @code blocks so they render properly

@code command reports verbatim everything between it and @endcode.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/dict.h

@@ -40,21 +40,21 @@
  * entries and finally av_dict_free() to free the dictionary
  * and all its contents.
  *
- * @code
- * AVDictionary *d = NULL;                // "create" an empty dictionary
- * av_dict_set(&d, "foo", "bar", 0);      // add an entry
- *
- * char *k = av_strdup("key");            // if your strings are already allocated,
- * char *v = av_strdup("value");          // you can avoid copying them like this
- * av_dict_set(&d, k, v, AV_DICT_DONT_STRDUP_KEY | AV_DICT_DONT_STRDUP_VAL);
- *
- * AVDictionaryEntry *t = NULL;
- * while (t = av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX)) {
- *     <....>                             // iterate over all entries in d
- * }
- *
- * av_dict_free(&d);
- * @endcode
+ @code
+   AVDictionary *d = NULL;           // "create" an empty dictionary
+   AVDictionaryEntry *t = NULL;
+
+   av_dict_set(&d, "foo", "bar", 0); // add an entry
+
+   char *k = av_strdup("key");       // if your strings are already allocated,
+   char *v = av_strdup("value");     // you can avoid copying them like this
+   av_dict_set(&d, k, v, AV_DICT_DONT_STRDUP_KEY | AV_DICT_DONT_STRDUP_VAL);
+
+   while (t = av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX)) {
+       <....>                             // iterate over all entries in d
+   }
+   av_dict_free(&d);
+ @endcode
  *
  */