.gitmodules: link to openzfs github repository

Update the test images link to reference the openzfs github repository.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Issue openzfs#11868

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "scripts/zfs-images"]
 	path = scripts/zfs-images
-	url = https://github.com/zfsonlinux/zfs-images
+	url = https://github.com/openzfs/zfs-images