update authentication text to show we allow login with gitlab and github (#272)

Author: nullfunc

docs/concepts/authentication.md

@@ -12,7 +12,7 @@ To do pretty much anything with Defang, you'll need to authenticate with the sys
 defang login
 ```
 
-This will prompt you to open a browser and log in to your [Defang account](/docs/concepts/accounts). For now, the only way to log in is with GitHub, though we will offer other providers to authenticate in the future. Once you've logged in, you can close the browser and return to the terminal. You should see a message that you've successfully logged in.
+This will prompt you to open a browser and log in to your [Defang account](/docs/concepts/accounts). You can log in using [GitHub](https://github.com/) or [GitLab](https://gitlab.com/). We will offer other authentication providers in the future. Once you've logged in, you can close the browser and return to the terminal. You will see a message that you've successfully logged in.
 
 :::tip
 Keep in mind that your Defang account is separate from your [cloud provider account](/docs/concepts/defang-byoc). You will need to authenticate with your cloud provider account separately to deploy services to your own cloud account.