bpo-40517: Implement syntax highlighting support for ASDL (#19928)

diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst
index fc04114..6c6ad01 100644
--- a/Doc/library/ast.rst
+++ b/Doc/library/ast.rst
@@ -35,7 +35,7 @@
 The abstract grammar is currently defined as follows:
 
 .. literalinclude:: ../../Parser/Python.asdl
-   :language: none
+   :language: asdl
 
 
 Node classes