[#10459] 📝 Update documentation to use yarn --cwd arg

Signed-off-by: LvffY <louberger@hotmail.fr>

microsite/blog/2020-04-30-how-to-quickly-set-up-backstage.md

@@ -102,8 +102,7 @@ Install in your app’s package folder (`<root>/packages/app`) with:
 
 ```bash
 # From your Backstage root directory
-cd packages/app
-yarn add @backstage/plugin-<plugin-name>
+yarn add --cwd packages/app @backstage/plugin-<plugin-name>
 ```
 
 After that, you inject the plugin into the application where you want it to be exposed. Please read the documentation for the specific plugin you are installing for more information.