contributing: clone wlroots fork with SSH

HTTPS URLs aren't writable.

CONTRIBUTING.md

@@ -14,7 +14,7 @@ don't, however, allow me to make a suggestion: feature branches pulled from
 upstream. Try this:
 
 1. Fork wlroots
-2. `git clone https://github.com/username/wlroots && cd wlroots`
+2. `git clone git@github.com:<username>/wlroots.git && cd wlroots`
 3. `git remote add upstream https://github.com/swaywm/wlroots`
 
 You only need to do this once. You're never going to use your fork's master