Revert unwanted changes.
diff --git a/Doc/library/language.rst b/Doc/library/language.rst
index bcf9ac0..7d6af7d 100644
--- a/Doc/library/language.rst
+++ b/Doc/library/language.rst
@@ -15,7 +15,7 @@
 .. toctree::
 
    parser.rst
-   ast.rst
+   _ast.rst
    symbol.rst
    token.rst
    keyword.rst