docs: update open sighnup docs

AdminForth/1852/use-only-afcl-buttons-across-p

Author: yaroslav8765

adminforth/documentation/docs/tutorial/09-Plugins/09-open-signup.md

@@ -36,6 +36,7 @@ new OpenSignupPlugin({
     defaultFieldValues: {
       role: 'user',
     },
+    expectedOrigin: process.env.OPEN_SIGHNUP_ORIGIN || 'http://localhost:3500',
   }),
 ```