bug #3891 Fixed wrong method call. (cmfcmf)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3891).

Discussion
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Fixed wrong method call.

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

Commits
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c1dce1e Changed wrong method call.

components/http_kernel/introduction.rst

@@ -658,7 +658,7 @@ argument as follows::
     // create some other request manually as needed
     $request = new Request();
     // for example, possibly set its _controller manually
-    $request->attributes->add('_controller', '...');
+    $request->attributes->set('_controller', '...');
 
     $response = $kernel->handle($request, HttpKernelInterface::SUB_REQUEST);
     // do something with this response