[3.12] Fix rendering of null character in ast.rst (GH-116080) (#116160)

Fix rendering of null character in ast.rst (GH-116080)
(cherry picked from commit c04a981)

Co-authored-by: Shantanu <[email protected]>

Author: miss-islington

Doc/library/ast.rst

@@ -2185,7 +2185,7 @@ and classes for traversing abstract syntax trees:
    ``await`` as variable names.  The lowest supported version is
    ``(3, 4)``; the highest is ``sys.version_info[0:2]``.
 
-   If source contains a null character ('\0'), :exc:`ValueError` is raised.
+   If source contains a null character (``\0``), :exc:`ValueError` is raised.
 
    .. warning::
       Note that successfully parsing source code into an AST object doesn't