Fix book building

[s373r]

Environment

• $ mdbook --version     
  mdbook v0.4.25

• Commit: the latest at the moment (33872fe)

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