rockspec: use git+https:// for git repository URL

GitHub is going to disable unencrypted Git protocol, so `git://` URLs
will stop working soon (see [1]).

[1]: https://github.blog/2021-09-01-improving-git-protocol-security-github/

Part of tarantool/tarantool#6587

websocket-scm-1.rockspec

@@ -2,7 +2,7 @@ package = "websocket"
 version = "scm-1"
 
 source = {
-    url = "git://github.com/tarantool/websocket.git",
+    url = "git+https://github.com/tarantool/websocket.git",
     branch = 'master',
 }