Add correct env to yarn run build command

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

docs/administration/index.md

@@ -73,7 +73,7 @@ yarn install
 
 Finally, we can build the front-end (this can take a few seconds)
 ```bash
-yarn run build
+NODE_ENV=production yarn run build
 ```
 
 Let's go back to the main directory