Update security/custom_authenticator.rst

Co-authored-by: Christian Flothmann <[email protected]>

Author: Spomky

security/custom_authenticator.rst

@@ -220,7 +220,7 @@ User Identifier
 ~~~~~~~~~~~~~~~
 
 The user identifier is a unique string that identifies the user. It is used
- to load the user using :ref:`the user provider <security-user-providers>`.
+to load the user using :ref:`the user provider <security-user-providers>`.
 This identifier is often something like the user's email address or username,
 but it could be any unique value associated with the user.