firewall: T6581: fix completion for "show firewall ... rule"

[dmbaturin]

Change Summary

Fix an off-by-one error that prevented completion from working.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Component(s) name

Firewall op mode.

Proposed changes

How to test

Create a firewall ruleset, see the completion for run show firewall name ... rule.

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

👍
No issues in unused-imports

[github-actions]

CI integration ❌ failed!

Details

CI logs

• CLI Smoketests ❌ failed
• Config tests 👍 passed
• RAID1 tests 👍 passed