Remove redundant braces in ArgGroup synopsis

[remkop]

Argument groups that are optional (multiplicity = "0..1") are shown in [ and ] square brackets in the synopsis.
Argument groups that are required (multiplicity = "1") are shown in ( and ) round brackets in the synopsis.

This may result in redundant round braces for groups or subgroups that have only a single option or positional parameter, and do not have subgroups themselves. For example:

@Command class Redundant {
    class GroupWithOneOption {
        @Option(names = "--option1", required = true) boolean value1;
    }

    class AllGroups {
        @ArgGroup(exclusive = false, multiplicity = "1") GroupWithOneOption group;
        @Option(names = "--option2", required = true) boolean value2;
    }

    @ArgGroup(exclusive = true, multiplicity = "1")
    AllGroups allGroups;
}

Gives a synopsis like this:

((--option) | --option2)

It would be more natural if the nested group (that only has one option and no subgroups) was shown without braces:

(--option | --option2)

Optional groups should not be impacted by this: the [ and ] square brackets are not redundant. (We can look at further improvements in the future.)