minor #10642 Corrected typo (obax)

This PR was submitted for the master branch but it was merged into the 4.1 branch instead (closes #10642).

Discussion
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Corrected typo

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Commits
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9f17c96 Update custom_authentication_provider.rst

Author: javiereguiluz

security/custom_authentication_provider.rst

@@ -14,7 +14,7 @@ How to Create a custom Authentication Provider
 
 Symfony provides support for the most
 :doc:`common authentication mechanisms </security/auth_providers>`. However, your
-app may need to integrated with some proprietary single-sing-on system or some
+app may need to integrated with some proprietary single-sign-on system or some
 legacy authentication mechanism. In those cases you could create a custom
 authentication provider. This article discusses the core classes involved
 in the authentication process, and how to implement a custom authentication