change help text to match embedded usage

[dceejay]

now that that is the default

• 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

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• 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 a0d079c on change-help-text-for-when-embedded
into ad6714b on master.