move cwd as the first argument to yarn

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

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

@@ -102,7 +102,7 @@ Install in your app’s package folder (`<root>/packages/app`) with:
 
 ```bash
 # From your Backstage root directory
-yarn add --cwd packages/app @backstage/plugin-<plugin-name>
+yarn --cwd packages/app add @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.