Fix release note headings

NEWS.rst

@@ -2,15 +2,15 @@ Release History
 ---------------
 
 0.5.1 (2016-01-13)
-~~~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^^^
 
 * Python 2.6 compatilibity restored (although 2.6 is still missing from the
   current CI configuration)
 
 * README converted back to reStructured Text formatting
 
 0.5.0 (2016-01-12)
-~~~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^^^
 
 * Updated to include all features from the Python 3.4 and 3.5 releases of
   contextlib (also includes some ``ExitStack`` enhancements made following
@@ -23,21 +23,21 @@ Release History
 
 
 0.4.0 (2012-05-05)
-~~~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^^^
 
 * Issue #8: Replace ContextStack with ExitStack (old ContextStack API
   retained for backwards compatibility)
 * Fall back to unittest2 if unittest is missing required functionality
 
 
 0.3.1 (2012-01-17)
-~~~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^^^
 
 * Issue #7: Add MANIFEST.in so PyPI package contains all relevant files
 
 
 0.3 (2012-01-04)
-~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^
 
 * Issue #5: ContextStack.register no longer pointlessly returns the wrapped
   function
@@ -53,14 +53,14 @@ Release History
 
 
 0.2 (2011-12-15)
-~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^
 
 * Renamed CleanupManager to ContextStack (hopefully before anyone started
   using the module for anything, since I didn't alias the old name at all)
 
 
 0.1 (2011-12-13)
-~~~~~~~~~~~~~~~~
+^^^^^^^^^^^^^^^^
 
 * Initial release as a backport module
 * Added CleanupManager (based on a `Python feature request`_)