minor #32100 Update ajax security cheat sheet link (steef)

This PR was merged into the 3.4 branch.

Discussion
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Update ajax security cheat sheet link

| Q             | A
| ------------- | ---
| Branch?       | 3.4 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

# Description
After looking in the `JsonResponse` class I found an old OWASP [link](https://www.owasp.org/index.php/AJAX_Security_Cheat_Sheet#Always_return_JSON_with_an_Object_on_the_outside). As the cheat sheet series project is moved to GitHub the link can be updated to the [following](https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/AJAX_Security_Cheat_Sheet.md).

## Screenshot
<img width="552" alt="Screenshot 2019-06-19 at 16 52 19" src="https://user-images.githubusercontent.com/34915382/59776184-a7e2bf80-92b2-11e9-8d23-802acfddfb1d.png">

Commits
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32d02d6 Update ajax security cheat sheet link

src/Symfony/Component/HttpFoundation/JsonResponse.php

@@ -18,7 +18,7 @@
  * object. It is however recommended that you do return an object as it
  * protects yourself against XSSI and JSON-JavaScript Hijacking.
  *
- * @see https://www.owasp.org/index.php/OWASP_AJAX_Security_Guidelines#Always_return_JSON_with_an_Object_on_the_outside
+ * @see https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/AJAX_Security_Cheat_Sheet.md#always-return-json-with-an-object-on-the-outside
  *
  * @author Igor Wiedler <igor@wiedler.ch>
  */