minor #21235 [Testing] Update a section heading to improve search results (javiereguiluz)

This PR was merged into the 6.4 branch.

Discussion
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[Testing] Update a section heading to improve search results

If you search for "testing HTTP headers" you don't get good results. Let's add `HTTP` explicitly to the heading of this section to try to improve this.

Commits
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609d93a [Testing] Update a section heading to improve search results

Author: javiereguiluz

testing.rst

@@ -745,8 +745,10 @@ a shortcut to make AJAX requests::
     // the required HTTP_X_REQUESTED_WITH header is added automatically
     $client->xmlHttpRequest('POST', '/submit', ['name' => 'Fabien']);
 
-Sending Custom Headers
-......................
+.. _sending-custom-headers:
+
+Sending Custom HTTP Headers
+...........................
 
 If your application behaves according to some HTTP headers, pass them as the
 second argument of ``createClient()``::