Edit SSH Certificate Extensions guide

Closes #22595

Indicate that the user certificate you are exporting would be the same
as the one you receive when authenticating to Teleport. Refer to your
existing user cert file instead using `tctl auth sign`.

docs/pages/admin-guides/management/guides/ssh-key-extensions.mdx

@@ -48,14 +48,23 @@ spec:
 
 ## Step 3/3. Issue a user certificate
 
-A user certificate may be issued with the following command, where `<USERNAME>` is the Teleport user to generate the SSH certificate for:
+When you authenticated to Teleport to assume the `developer` role we defined
+earlier, Teleport added the certificate extensions you configured to your user
+certificate. You can then refer to this path in your local SSH configuration.
+
+For convenience, open a terminal and assign an environment variable to the
+location of your user certificate. Replace <Var name="example.teleport.sh"> with
+the domain name of your Teleport cluster and <Var name="myuser"> with the name
+of your Teleport user:
+
 ```code
-$ tctl auth sign --out out.cer --user=<USERNAME>
+$ export TELEPORT_CERT=~/.tsh/keys/<Var name="example.teleport.sh">/<Var name="myuser">-ssh/<Var name="example.teleport.sh">-cert.pub
 ```
 
 To test that authentication with this signed certificate is working correctly, SSH into `github.com` with your organization's user:
+
 ```code
-$ ssh -i out.cer org-<ID>@github.com
+$ ssh -i $TELEPORT_CERT org-<ID>@github.com
 ```
 If authentication is successful,  a "You've successfully authenticated" message should be displayed in the terminal.
 
@@ -64,7 +73,7 @@ This newly generated certificate may then be used when interacting with GitHub o
 ```code
 Host github.com
  HostName github.com
- IdentityFile path/to/out.cer
+ IdentityFile path/to/cert.pub
 ```
 
 When using SSH Certificate Authorities, you should retrieve your GitHub repository's SSH URL from the GitHub UI so the correct SSH user is used for authentication. For more information, see [About SSH URLs with SSH certificates](https://docs.github.com/en/organizations/managing-git-access-to-your-organizations-repositories/about-ssh-certificate-authorities#about-ssh-urls-with-ssh-certificates).