bbb-lti: Fixed bug in nginx configuration that prevented https of working fine

bbb-lti/lti.nginx

@@ -4,7 +4,7 @@
            proxy_pass         http://127.0.0.1:8080;
            proxy_redirect     default;
            proxy_set_header   X-Forwarded-For   $proxy_add_x_forwarded_for;
-           proxy_set_header   X-Forwarded-Proto $http_x_forwarded_proto;
+           proxy_set_header   X-Forwarded-Proto $scheme;
 
            # Allow 30M uploaded presentation document.
            client_max_body_size       30m;