Fix hub variable for jupyterhub_dashboard

[kenafoster]

Reference Issues or PRs

Fixes JupyterHub Dashboard issue - #2591

What does this implement/fix?

Put a x in the boxes that apply

• [ X ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• [ X ] Did you test the pull request locally?
• Did you add new tests?

How to test this PR?

Deploy Nebari from this branch. Make sure at least one JupyterLab user server is running, then check the Grafana dashboard 'Main > JupyterHub Dashboard'. Several dashboards which were empty prior to this change should now show data.

Any other comments?