updating whatsnew docs

Author: ArkadeepAdhikari

doc/source/user_guide/indexing.rst

@@ -237,7 +237,7 @@ new column. In 0.21.0 and later, this will raise a ``UserWarning``:
 
     In [1]: df = pd.DataFrame({'one': [1., 2., 3.]})
     In [2]: df.two = [4, 5, 6]
-    UserWarning: Pandas doesn't allow Series to be assigned into nonexistent columns - see :ref:`attribute accessing for Series <attribute.access>`
+    UserWarning: Pandas doesn't allow Series to be assigned into nonexistent columns - see :ref:`attribute accessing for Series <indexing.attribute_access>`
     In [3]: df
     Out[3]:
        one