Required ArgGroup displayed incorrenctly

[panicbit]

The last character in the usage message of ArgGroups seems to always be missing.

For example this code

App::new("demo")
    .arg(Arg::with_name("foo").long("foo"))
    .arg(Arg::with_name("bar").long("bar"))
    .group(ArgGroup::with_name("options").required(true).args(&[
        "foo", "bar"
    ]))
    .get_matches();

displays the group as [--foo|--ba]

[kbknapp]

As I said in the PR, thanks for finding, filing, and fixing this! 👍