Added missing Permission class to example

Following the example will give the following error;

```[Emergency] Uncaught Error: Class {my namespace}\Permission not found```

Added the missing class

docs/en/02_Developer_Guides/09_Security/02_Permissions.md

@@ -23,6 +23,7 @@ The simple usage, Permission::check("PERM_CODE") will detect if the currently lo
 This method should return a map of permission code names with a human readable explanation of its purpose.
 
 ```php
+use SilverStripe\Security\Permission;
 use SilverStripe\Security\PermissionProvider;
 use SilverStripe\Security\Security;