Add correct usage for firewall rules

[optik-aper]

Description

Fixes command usage for the firewall rules

Related Issues

Closes #411

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[github-actions]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v3	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[mondragonfx]

Looks good.