opt: Fix the documentation mentioning av_set_string3

It is av_opt_set now.

libavutil/opt.h

@@ -319,7 +319,7 @@ void av_opt_set_defaults(void *s);
  * @return the number of successfully set key/value pairs, or a negative
  * value corresponding to an AVERROR code in case of error:
  * AVERROR(EINVAL) if opts cannot be parsed,
- * the error code issued by av_set_string3() if a key/value pair
+ * the error code issued by av_opt_set() if a key/value pair
  * cannot be set
  */
 int av_set_options_string(void *ctx, const char *opts,
@@ -408,7 +408,7 @@ int av_opt_eval_q     (void *obj, const AVOption *o, const char *val, AVRational
  *         was found.
  *
  * @note Options found with AV_OPT_SEARCH_CHILDREN flag may not be settable
- * directly with av_set_string3(). Use special calls which take an options
+ * directly with av_opt_set(). Use special calls which take an options
  * AVDictionary (e.g. avformat_open_input()) to set options found with this
  * flag.
  */