Update stdlib/@tests/stubtest_allowlists/py39.txt

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: srittau

stdlib/@tests/stubtest_allowlists/py39.txt

@@ -45,7 +45,7 @@ tempfile.SpooledTemporaryFile.writable
 tkinter.Tk.split  # Exists at runtime, but missing from stubs
 
 # Will always raise. Not included to avoid type checkers inferring that
-# TypeAliasType instances are callable.
+# Sentinel instances are callable.
 typing_extensions.Sentinel.__call__