Fix Git reference to OpenZFS repository

This change fixes the Git reference to the OpenZFS repository now that
GitHub has deprecated the original Git protocol.

.gitmodules

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