Fix docs homepage

[michalrzak]

Right now the homepage of the documentation is not building correctly (see Project Structure section: https://juliareinforcementlearning.org/docs/)

I am pretty sure this is just due to an unmatched <!-- ``` -->. This PR hopefully fixes this.

Note however that I did not build the docs myself to verify.

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[jeremiahpslewis]

Thanks!