changes assign to a reference on method in dsintro

Author: datajanko

doc/source/dsintro.rst

@@ -518,7 +518,7 @@ DataFrame is returned, with the new values inserted.
                    D=lambda x: x['A'] + x['C'])
 
   This may subtly change the behavior of your code when you're
-  using ``.assign()`` to update an existing column. Prior to Python 3.6,
+  using :func:`DataFrame.assign` to update an existing column. Prior to Python 3.6,
   callables referring to other variables being updated would get the "old" values
 
 Indexing / Selection