fix(A8:2019): remove external reference

Keep markdown source coherent with distributable formats.

Author: PauloASilva

2019/en/src/0xa8-injection.md

@@ -123,12 +123,10 @@ and **RASP** solutions may help to detect these issues on live systems.
 
 * [CWE-77: Command Injection][5]
 * [CWE-89: SQL Injection][6]
-* [HOW TO: Command Injection, HackerOne][7]
 
 [1]: https://www.owasp.org/index.php/Injection_Flaws
 [2]: https://www.owasp.org/index.php/SQL_Injection
 [3]: https://www.owasp.org/images/e/ed/GOD16-NOSQL.pdf
 [4]: https://www.owasp.org/index.php/Command_Injection
 [5]: https://cwe.mitre.org/data/definitions/77.html
 [6]: https://cwe.mitre.org/data/definitions/89.html
-[7]: https://www.hackerone.com/blog/how-to-command-injections