docs: improve docs in Admin guide (#1794)

[skip ci]

docs/Admin-Guide.md

@@ -5,10 +5,11 @@ This guide is for instance admins who may be having trouble with Pinafore compat
 
 By default, [Mastodon allows cross-origin access to the `/api` endpoint](https://github.com/tootsuite/mastodon/blob/50529cbceb84e611bca497624a7a4c38113e5135/config/initializers/cors.rb#L15-L20). Thus Pinafore should "just work" for most Mastodon servers.
 
-If the nginx/Apache settings have been changed, though, then Pinafore might not be able to connect to an instance. To check if the instance is supported, run this command:
+If the nginx/Apache settings have been changed, though, then Pinafore might not be able to connect to an instance. To check if the instance is supported, run this command (replacing `myinstance.com` with your instance URL):
 
 ```bash
-curl -sLv example.com/api/v1/instance -o /dev/null
+curl -sLv -H 'Origin: https://pinafore.social' -o /dev/null \
+  myinstance.com/api/v1/instance
 ```
 
 If you see this in the output: