Fix double negative typo in JupyterLab module error message

[blinkagent]

The JupyterLab module's run.sh has a typo in the error message when no valid installer is found:

echo "No valid installer is not installed"

This double negative reads oddly. It should be:

echo "No valid installer is installed"

or:

echo "A valid installer is not installed"

https://github.com/coder/registry/blob/main/registry/coder/modules/jupyterlab/run.sh#L17

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Created on behalf of @ericpaulsen