CONTRIBUTING: Replace outdated image link

The screenshot for fork button, which was available from tinypic, is not
available now.  This commit replaces it with other image from the Github
Docs site.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>

CONTRIBUTING.md

@@ -5,7 +5,7 @@ If you want to contribute to [linux-insides](https://github.com/0xAX/linux-insid
 
 1. Press the fork button:
 
-    ![fork](http://oi58.tinypic.com/jj2trm.jpg)
+    ![fork](https://docs.github.com/assets/images/help/repository/fork_button.jpg)
 
 2. Clone the repository from your account with: