Change Zephyr URL to https

Using https allows us to clone without being signed into GitHub

Change-Id: Ia24762d8c5b1c61892720807bcc9b8e76f9e8edf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/zephyr/+/206215
Commit-Queue: Taylor Cramer <cramertj@google.com>
Commit-Queue: Yuval Peress <peress@google.com>
Pigweed-Auto-Submit: Yuval Peress <peress@google.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>