minor #5071 Fixed incorrect plural form (Katharina Störmer)

This PR was merged into the 2.3 branch.

Discussion
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Fixed incorrect plural form

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
-------

6ea16f0 Fixed incorrect plural form

book/testing.rst

@@ -387,7 +387,7 @@ or perform more complex requests. Some useful examples::
         array('photo' => $photo)
     );
 
-    // Perform a DELETE requests and pass HTTP headers
+    // Perform a DELETE request and pass HTTP headers
     $client->request(
         'DELETE',
         '/post/12',