fix typo and add ref tag in whatsnew

Author: nateyoder

doc/source/whatsnew/v0.20.0.txt

@@ -92,6 +92,8 @@ Backwards incompatible API changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 
+.. _whatsnew.api_breaking.index_map
+
 Map on Index types now return other Index types
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -133,7 +135,7 @@ Map on Index types now return other Index types
       mi.map(lambda x: x[0])
 
 
--  ``map`` on a Series withe datetime64 values may return int64 dtypes rather than int32
+-  ``map`` on a Series with datetime64 values may return int64 dtypes rather than int32
 
   .. ipython:: python