gh-99735: Use required=True in argparse subparsers example (GH-100927)

(cherry picked from commit 6aab56f3c2)

Co-authored-by: Patricio Paez <nospam@pp.com.mx>

Doc/library/argparse.rst

@@ -1854,7 +1854,7 @@ Sub-commands
      ...
      >>> # create the top-level parser
      >>> parser = argparse.ArgumentParser()
-     >>> subparsers = parser.add_subparsers()
+     >>> subparsers = parser.add_subparsers(required=True)
      >>>
      >>> # create the parser for the "foo" command
      >>> parser_foo = subparsers.add_parser('foo')