Fix typo in usage message

[kazuhitoyokoi]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I found that usage message lacks ']' in command line. Therefore, I added the character in the code.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 77.607% when pulling bb5f777 on kazuhitoyokoi:master-fixtypo into d174103 on node-red:master.

[coveralls]

Coverage remained the same at 77.607% when pulling bb5f777 on kazuhitoyokoi:master-fixtypo into d174103 on node-red:master.