Updated version note for Font.outline to 2.5.7.

2.5.6 released while this was still in review.

Author: cherez

docs/reST/ref/font.rst

@@ -340,7 +340,7 @@ solves no longer exists, it will likely be removed in the future.
               font.outline = old_outline
               return outlined_text_surf
 
-      .. versionadded:: 2.5.6
+      .. versionadded:: 2.5.7
 
       .. ## Font.outline ##