change help text to match embedded usage

[dceejay]

now that that is the default

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

Proposed changes

As we now build in the admin feature the help text should reflect that more than the old cli usage - so should say node-red admin rather than node-red-admin.

Checklist

• [x] I have read the contribution guidelines
• [ ] For non-bugfix PRs, I have discussed this change on the forum/slack team.
• [x] I have run npm run test to verify the unit tests pass
• [ ] I have added suitable unit tests to cover the new/changed functionality

[coveralls]

coverage: 81.27%. remained the same when pulling a0d079cf2ab243a6d4d9593f40470c5d05564065 on change-help-text-for-when-embedded into ad6714b938c033b28f003894061b2ebf547f8960 on master.