minor #3821 Update custom_authentication_provider.rst (leberknecht)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3821).

Discussion
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Update custom_authentication_provider.rst

improved description of the getKey method

Commits
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89c2d88 Update custom_authentication_provider.rst

cookbook/security/custom_authentication_provider.rst

@@ -346,7 +346,7 @@ requires the following methods:
   and ``remember_me`` and defines the position at which the provider is called;
 
 * ``getKey`` method which defines the configuration key used to reference
-  the provider;
+  the provider in the firewall configuration;
 
 * ``addConfiguration`` method, which is used to define the configuration
   options underneath the configuration key in your security configuration.