Fix trailing slash

POSTINSTALL.md

@@ -1,5 +1,8 @@
 This app integrates with Cloudron SSO. It allows any logged in Cloudron user 
-to access the WordPress dashboard. Any administator on the Cloudron is 
-automatically made an administrator in WordPress. Non-administrators get
-the default `editor` role in WordPress.
+to access the WordPress dashboard. Cloudron administrators are automatically 
+made WordPress administrators. Non-administrators get the `editor` role by
+default.
+
+You can access the dashboard directly at `/wp-admin/' (trailing slash is
+important!).