Added ldap to the list of user providers

Ldap is the new user provider in  Symfony 2.8, so it would be good to mention it in the list of all user providers.

cookbook/security/custom_provider.rst

@@ -9,7 +9,8 @@ When a user submits a username and password, the authentication layer asks
 the configured user provider to return a user object for a given username.
 Symfony then checks whether the password of this user is correct and generates
 a security token so the user stays authenticated during the current session.
-Out of the box, Symfony has an "in_memory" and an "entity" user provider.
+Out of the box, Symfony has four user providers: ``in_memory``, ``entity``, 
+``ldap`` and ``chain``.
 In this entry you'll see how you can create your own user provider, which
 could be useful if your users are accessed via a custom database, a file,
 or - as shown in this example - a web service.