fixes dsintro entry for assign

- improved example in warning
- linked reference to docstring of assign

Author: datajanko

doc/source/dsintro.rst

@@ -507,13 +507,15 @@ of one argument to be called on the ``DataFrame``. A *copy* of the original
 DataFrame is returned, with the new values inserted.
 
 .. warning::
-  Starting from Python 3.6 ``**kwargs`` is an ordered dictionary and :func`DataFrame.assign`
+  Starting from Python 3.6 ``**kwargs`` is an ordered dictionary and :func:`DataFrame.assign`
   respects the order of the keyword arguments. You may now use assign in the following way:
 
   .. ipython:: python
 
-      In [1]: df.assign(C = lambda x: x['A'] + x['B'],
-                        D = lambda x: x['A'] + x['C'])
+    df_warn = pd.DataFrame({"A": [1, 2, 3],
+                            "B": [4, 5, 6]})
+    df_warn.assign(C=lambda x: x['A'] + x['B'],
+                   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,