Update ldap.rst

Without the tags I was getting an error about the ldap service not being found and "Did you maybe forget to add the "ldap" service tag to this service?"

Author: mickadoo

security/ldap.rst

@@ -70,6 +70,8 @@ An LDAP client can be configured using the built-in
         services:
             Symfony\Component\Ldap\Ldap:
                 arguments: ['@Symfony\Component\Ldap\Adapter\ExtLdap\Adapter']
+                tags:
+                    - ldap
             Symfony\Component\Ldap\Adapter\ExtLdap\Adapter:
                 arguments:
                     -   host: my-server