Fix link to fork "button"

A /fork leads to link on the GitHub webview of the markdown,
which ends with mermaid-cli/blob/master/fork. This is the link 
to a directory called "fork" in root of the repository.
So go back to parent directory, two times, to get mermaid-cli/fork,
which is the link to the fork "button" of the current repository.

Author: BaumiCoder

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing
 
-[fork]: /fork
+[fork]: ../../fork
 [pr]: /compare
 [style]: https://standardjs.com/
 [code-of-conduct]: CODE_OF_CONDUCT.md