Issue #26462: Doc: avoid literal_block warnings, fix syntax highlighting.

Patch by Julien Palard.
diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst
index cf0c32e..6d5855b 100644
--- a/Doc/library/ast.rst
+++ b/Doc/library/ast.rst
@@ -113,6 +113,7 @@
 The abstract grammar is currently defined as follows:
 
 .. literalinclude:: ../../Parser/Python.asdl
+   :language: none
 
 
 :mod:`ast` Helpers