minor #12881 [Testing] Fix broken link (MrYamous)

This PR was merged into the 5.0 branch.

Discussion
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[Testing] Fix broken link

Fix a broken link here : https://symfony.com/doc/current/testing.html#ajax-requests

Symfony/Component/BrowserKit/Client has been replaced by Symfony/Component/BrowserKit/AbstractBrowser in 5.0

Commits
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0117fe5 Fix broken link

Author: javiereguiluz

testing.rst

@@ -488,7 +488,7 @@ script::
 AJAX Requests
 ~~~~~~~~~~~~~
 
-The Client provides a :method:`Symfony\\Component\\BrowserKit\\Client::xmlHttpRequest`
+The Client provides a :method:`Symfony\\Component\\BrowserKit\\AbstractBrowser::xmlHttpRequest`
 method, which has the same arguments as the ``request()`` method, and it's a
 shortcut to make AJAX requests::