Fix book building

# Environment - ```sh $ mdbook --version mdbook v0.4.25 ``` - Commit: the latest at the moment ([33872fe](https://github.com/amethyst/rustrogueliketutorial/commit/33872fe582f226178436847e1f74eafcbf9c0d1a)) # Without this fix ``` $ mdbook build 2023-01-02 22:07:10 [WARN] (mdbook::book): The output.html.google-analytics field has been deprecated; it will be removed in a future release. Consider placing the appropriate site tag code into the theme/head.hbs file instead. The tracking code may be found in the Google Analytics Admin page. 2023-01-02 22:07:10 [INFO] (mdbook::book): Book building has started 2023-01-02 22:07:10 [INFO] (mdbook::book): Running the html backend 2023-01-02 22:07:10 [ERROR] (mdbook::utils): Error: Rendering failed 2023-01-02 22:07:10 [ERROR] (mdbook::utils): Caused By: theme dir /home/s373r/src/rust/rustrogueliketutorial/book/rust does not exist ``` # After this fix ``` $ mdbook build 2023-01-02 22:16:02 [INFO] (mdbook::book): Book building has started 2023-01-02 22:16:02 [INFO] (mdbook::book): Running the html backend ```