fix typo in config name

Author: Tobion

README.md

@@ -98,8 +98,8 @@ playing with it, you can remove it by following these steps:
 
   * remove the `web/bundles/acmedemo` directory;
 
-  * remove the `security.providers`, `security.firewall.login` and
-    `security.firewall.secured_area` entries in the `security.yml` file or
+  * remove the `security.providers`, `security.firewalls.login` and
+    `security.firewalls.secured_area` entries in the `security.yml` file or
     tweak the security configuration to fit your needs.
 
 What's inside?