minor #15022 [Testing] Fixed small typo (ArSn)

This PR was merged into the 5.2 branch.

Discussion
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[Testing] Fixed small typo

Commits
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dde9669 Fixed small typo

Author: javiereguiluz

testing.rst

@@ -607,7 +607,7 @@ submitting a login form - make a test very slow. For this reason, Symfony
 provides a ``loginUser()`` method to simulate logging in in your functional
 tests.
 
-Instead of login in with real users, it's recommended to create a user only for
+Instead of logging in with real users, it's recommended to create a user only for
 tests. You can do that with Doctrine :ref:`data fixtures <user-data-fixture>`,
 to load the testing users only in the test database.