fix documentation security.yml

[alfonsodev]

as described here
#2855
there is a mistake in the documentation for configure.yml
this PR just fixes that line.

[xabbuh]

But the service is indeed defined here: https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/config/security.xml#L49

[alfonsodev]

I had the exact same issues as reported in #2855 with symfony 4

  The service "security.authentication.provider.dao.main" has a dependency on  
   a non-existent service "fos_user.user_checker".                             

And I assumed because the issue was closed the solution exposed there was the right one, it did work for me at least the initial setup, register, log in, and log out. I didn't test all the features.

What would then be the right setup for security.yml to work with Symfony 4 ?

the version being used is

        "friendsofsymfony/user-bundle": "^2.1",

[XWB]

It should be fos_user.user_checker as defined in https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/config/security.xml#L49