bug #6892 Update custom_provider.rst (asandjivy)

This PR was submitted for the 3.1 branch but it was merged into the 2.8 branch instead (closes #6892).

Discussion
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Update custom_provider.rst

"in_memory" does not exist. This is a just a provider name. The real keyword is "memory"

Commits
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2d5b420 Update custom_provider.rst

Author: weaverryan

security/custom_provider.rst

@@ -9,7 +9,7 @@ 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 four user providers: ``in_memory``, ``entity``, 
+Out of the box, Symfony has four user providers: ``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.