Fix error messages in argparse examples (GH-6435)

Fix error messages in argparse examples (GH-6435) · python/cpython@04e8293

Original file line number	Diff line number	Diff line change
`@@ -712,7 +712,7 @@ be positional::`
`712`	`712`	`Namespace(bar='BAR', foo='FOO')`
`713`	`713`	`>>> parser.parse_args(['--foo', 'FOO'])`
`714`	`714`	`usage: PROG [-h] [-f FOO] bar`
`715`		`- PROG: error: too few arguments`
	`715`	`+ PROG: error: the following arguments are required: bar`
`716`	`716`
`717`	`717`
`718`	`718`	`action`
`@@ -898,7 +898,7 @@ values are:`
`898`	`898`	`Namespace(foo=['a', 'b'])`
`899`	`899`	`>>> parser.parse_args([])`
`900`	`900`	`usage: PROG [-h] foo [foo ...]`
`901`		`- PROG: error: too few arguments`
	`901`	`+ PROG: error: the following arguments are required: foo`
`902`	`902`
`903`	`903`	.. _`argparse.REMAINDER`:
`904`	`904`