[3.9] bpo-40939: Fix test_keyword for the old parser (GH-20814)

diff --git a/Lib/pydoc.py b/Lib/pydoc.py
index a5368bf..35ef3eb 100755
--- a/Lib/pydoc.py
+++ b/Lib/pydoc.py
@@ -1817,7 +1817,7 @@
         'False': '',
         'None': '',
         'True': '',
-        '__new_parser__': '',
+        '__peg_parser__': '',
         'and': 'BOOLEAN',
         'as': 'with',
         'assert': ('assert', ''),