DOC: fixes references in indexing and whatsnew

Author: deniederhut

Diff for: doc/source/indexing.rst

@@ -271,7 +271,7 @@ 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 https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access
+    UserWarning: Pandas doesn't allow Series to be assigned into nonexistent columns - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute_access
     In[3]: df
     Out[3]:
        one

Diff for: doc/source/whatsnew/v0.21.0.txt

@@ -61,10 +61,10 @@ using the :func:`to_numeric` function (or :func:`to_datetime`, :func:`to_timedel
    df['C'] = pd.to_numeric(df['C'], errors='coerce')
    df.dtypes
 
-.. _whatsnew_0210.enhancements.column-creation:
+.. _whatsnew_0210.enhancements.attribute_access:
 
 Improved warnings when attempting to create columns
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 New users are often flummoxed by the relationship between column operations and attribute
 access on ``DataFrame`` instances (:issue:`5904` & :issue:`7175`). Two specific instances
@@ -104,7 +104,7 @@ This does not permit that column to be accessed as an attribute:
   1  2.0  7.0
   2  3.0  9.0>
 
-Both of these now raise a ``UserWarning`` about the potential for unexpected behavior. See `Attribute Access <https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access>`__.
+Both of these now raise a ``UserWarning`` about the potential for unexpected behavior. See :ref:`Attribute Access <indexing.attribute_access>`.
 
 .. _whatsnew_0210.enhancements.other: