Use gitlab.com in 2FA recovery ssh command

Most of the times the users following a recovery guide are the ones that
use GitLab.com (SaaS), because on self-managed instances they are more
likely to reach out to their local administrators.

doc/user/profile/account/two_factor_authentication.md

@@ -393,9 +393,13 @@ a new set of recovery codes with SSH:
 1. Run:
 
    ```shell
-   ssh git@gitlab.example.com 2fa_recovery_codes
+   ssh git@gitlab.com 2fa_recovery_codes
    ```
    
+   NOTE:
+   On self-managed instances, replace **`gitlab.com`** in the command above
+   with the GitLab server hostname (`gitlab.example.com`).
+
 1. You are prompted to confirm that you want to generate new codes.
    Continuing this process invalidates previously saved codes: