fix help message with @OptionGroup ParsableCommand

[monjara]

Fixes #578. This PR fixes usage text generated by command with ParsableCommand annotated @OptionGroup. I have fixed it so that DecodedArguments created from OptionGroup are not recognized as ParsableCommand.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary