Fix typo in json_object_get_value_at documentation (#587)

plugins/include/json.inc

@@ -594,10 +594,8 @@ native json_object_get_name(const JSON:object, index, buffer[], maxlen);
  *
  * @param object            Object handle
  * @param index             Position from which get key name
- * @param buffer            Buffer to copy string to
- * @param maxlen            Maximum size of the buffer
  *
- * @return                  The number of cells written to the buffer
+ * @return                  JSON handle, Invalid_JSON if error occurred
  * @error                   If passed handle is not a valid object
  */
 native JSON:json_object_get_value_at(const JSON:object, index);