Update first_steps.md

Yet another revision, adding a warning to step three.

docs/first_steps.md

@@ -35,6 +35,8 @@ certbot certonly \
     --email you@example.org \
     --agree-tos
 ```
+
+**Remember to replace the example.org domain with your own domain, this command will not work if you dont.**
     
 4\. Create hard links to the full path of the new certificates. Assuming you are still in the mailcow root folder:
 ``` bash