minor #15567 [Security] Update ldap.rst (mickadoo)

This PR was merged into the 5.3 branch.

Discussion
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[Security] Update ldap.rst

## Overview

Without the tags I was getting this error:

> Cannot check credentials using the "Symfony\Component\Ldap\Ldap" ldap service, as such service is not found. Did you maybe forget to add the "ldap" service tag to this service?

Commits
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a5df2c4 Update ldap.rst

Author: javiereguiluz

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