bpo-36645: Fix ambiguous formatting in re.sub() documentation (GH-12879)

(cherry picked from commit 5ebfa84)

Co-authored-by: mollison <[email protected]>

Author: miss-islington

Doc/library/re.rst

@@ -902,6 +902,7 @@ form.
       Unknown escapes in *repl* consisting of ``'\'`` and an ASCII letter
       now are errors.
 
+   .. versionchanged:: 3.7
       Empty matches for the pattern are replaced when adjacent to a previous
       non-empty match.