update upstream tock url to use https protocol

When the upstream url is prefaced by git://, git fetch upstream
hangs and fails with

    fatal: unable to connect to github.com:
    github.com[0: 140.82.114.3]: errno=Operation timed out

but succeeds when the upstream url is prefaced by https://. This
is likely because Github no longer supports the git:// protocol.

.github/CONTRIBUTING.md

@@ -85,7 +85,7 @@ out your copy locally.
 ```text
 $ git clone [email protected]:username/tock.git
 $ cd tock
-$ git remote add upstream git://github.com/tock/tock.git
+$ git remote add upstream https://github.com/tock/tock.git
 ```
 
 ### Step 2: Branch