Correct the code example in Python 3.7's What's New (GH-5696)

There was an extra dash in the example for re.sub().
(cherry picked from commit b65cb16)

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

Author: miss-islington

Doc/whatsnew/3.7.rst

@@ -1157,7 +1157,7 @@ Changes in the Python API
 
   :func:`re.sub()` now replaces empty matches adjacent to a previous
   non-empty match.  For example ``re.sub('x*', '-', 'abxd')`` returns now
-  ``'-a-b--d-'`` instead of ``'-a-b--d-'`` (the first minus between 'b' and
+  ``'-a-b--d-'`` instead of ``'-a-b-d-'`` (the first minus between 'b' and
   'd' replaces 'x', and the second minus replaces an empty string between
   'x' and 'd').