Fix "Development Dependencies Install" guide [ci skip]

Since we use [workspaces](https://yarnpkg.com/lang/en/docs/workspaces/)
`yarn install` from the root should install all the javascript dependencies.

guides/source/development_dependencies_install.md

@@ -227,7 +227,6 @@ If you're using another database, check the file `activerecord/test/config.yml`
 If you installed Yarn, you will need to install the javascript dependencies:
 
 ```bash
-$ cd activestorage
 $ yarn install
 ```