update git submodule to use https protocol

github is dropping support for unauthenticated git protocol, as
announced on
https://github.blog/2021-09-01-improving-git-protocol-security-github/

.gitmodules

 [submodule "uritemplate-test"]
 	path = uritemplate-test
-	url = git://github.com/uri-templates/uritemplate-test.git
+	url = https://github.com/uri-templates/uritemplate-test.git