Fix link to the JupyterLite example file in the documentation

[jtpio]

Description

Follow-up to #45

In #45 it was noticed that the badge linking to the example file does not seem to be formatted correctly on ReadTheDocs:

Reproduce

This can be seen on the documentation website: https://jupyterlab-plugin-playground.readthedocs.io/en/latest/lite/lab

Expected behavior

It should be possible to click the badge and use the extension and open JupyterLite in a new tab.

Context

#45

[krassowski]

The badge in README does not seem to work right now either:

[jtpio]

Will open a PR to update some jupyterlite dependencies to the latest versions to see if that can help already.