Switch to https submodule

This change updates the reference to the ZFS submodule to use HTTPS as
opposed to SSH access, which is not supported for unauthenticated users.

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "zfs"]
 	path = zfs
-	url = [email protected]:openzfs/zfs.git
+	url = https://github.com/openzfs/zfs.git
 
 [submodule "linux"]
 	path = linux