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>

.gitmodules

diff --git a/.gitmodules b/.gitmodules
index dd4552c..3d10c7a 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -18,5 +18,5 @@
 	branch = main
 [submodule "third_party/zephyr"]
 	path = third_party/zephyr
-	url = git@github.com:zephyrproject-rtos/zephyr.git
+	url = https://github.com/zephyrproject-rtos/zephyr.git
 	branch = main