Error: ENOENT: no such file or directory, stat '/Users/username/typescript-book/_book/gitbook/gitbook-plugin-adsense/plugin.js'

when I follow the guide in https://github.com/basarat/typescript-book/blob/master/CONTRIBUTING.md and run the command `gitbook serve .` I see this error:

```bash
➜ typescript-book git:(master) gitbook serve .
Live reload server started on port: 35729
Press CTRL+C to quit ...

info: 11 plugins are installed
info: loading plugin "edit-link"... OK
info: loading plugin "github"... OK
info: loading plugin "adsense"... OK
info: loading plugin "header"... OK
info: loading plugin "livereload"... OK
info: loading plugin "highlight"... OK
info: loading plugin "search"... OK
info: loading plugin "lunr"... OK
info: loading plugin "sharing"... OK
info: loading plugin "fontsettings"... OK
info: loading plugin "theme-default"... OK
info: found 125 pages
info: found 139 asset files

Error: ENOENT: no such file or directory, stat '/Users/username/typescript-book/_book/gitbook/gitbook-plugin-adsense/plugin.js'
```

Read CONTRIBUTING.md and reproduce the issue by running `gitbook serve .` from the repository root. Investigate why the command looks for `_book/gitbook/gitbook-plugin-adsense/plugin.js`; done means the guide's serve command completes without the ENOENT error.

Written by the indexing model from the issue text.