users-guide: Enlarge title underlines in 8.8 release notes

Somehow the level-2 headings were all missing a tilde, causing Sphinx to complain.

docs/users_guide/8.8.1-notes.rst

@@ -20,7 +20,7 @@ Full details
 ------------
 
 Language
-~~~~~~~
+~~~~~~~~
 
 - :extension:`ScopedTypeVariables`: The type variable that a type signature on
   a pattern can bring into scope can now stand for arbitrary types. Previously,
@@ -28,27 +28,27 @@ Language
   unnecessary in `GHC proposal #29 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0029-scoped-type-variables-types.rst>`__. Also see :ghc-ticket:`15050`.
 
 Compiler
-~~~~~~~
+~~~~~~~~
 
 
 Runtime system
-~~~~~~~~~~~~~
+~~~~~~~~~~~~~~
 
 
 Template Haskell
-~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~
 
 
 ``ghc`` library
-~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~
 
 
 ``base`` library
-~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~
 
 
 Build system
-~~~~~~~~~~~
+~~~~~~~~~~~~
 
 
 Included libraries