Fix extension NAME with hyphens error message

[cllns]

rubocop-extension-generator rubocop-my-example

Will show an error:

Usage: rubocop-extension-generator NAME

The NAME must start with rubocop-, like rubocop-rspec.

In this case, the problem is that it should be rubocop-my_example (to fit RubyGems guidelines for naming gems), not that it doesn't begin with rubocop-.

I see two options:

1. Detecting this mistake and having a different error message for that case
2. Just adding a line about this issue to the existing error message, to cover both cases