minor symfony#10776 Update user_provider.rst (obax)

This PR was submitted for the 4.2 branch but it was merged into the 4.1 branch instead (closes symfony#10776).

Discussion
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Update user_provider.rst

Fixed a small typo

Commits
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5e61762 Update user_provider.rst

Author: javiereguiluz

security/user_provider.rst

@@ -14,7 +14,7 @@ that has two jobs:
 **Load the User for some Feature**
     Some features, like ``switch_user``, ``remember_me`` and many of the built-in
     :doc:`authentication providers </security/auth_providers>`, use the user provider
-    to load a User object via is "username" (or email, or whatever field you want).
+    to load a User object via its "username" (or email, or whatever field you want).
 
 Symfony comes with several built-in user providers: