minor symfony#16164 [Testing] Fix grammar issue (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
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[Testing] Fix grammar issue

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Commits
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aec97fd [Testing] Fix grammar issue

Author: javiereguiluz

testing.rst

@@ -573,7 +573,7 @@ will no longer be followed::
 Making AJAX Requests
 ....................
 
-The client provides a
+The client provides an
 :method:`Symfony\\Component\\BrowserKit\\AbstractBrowser::xmlHttpRequest`
 method, which has the same arguments as the ``request()`` method and is
 a shortcut to make AJAX requests::