Add specurl for incumbent settings object tracking in Promises

This is needed for Wattsi to generate an MDN compatibility box for the
HTML specification.

HTML PR: whatwg/html#5722

javascript/builtins/Promise.json

@@ -377,6 +377,7 @@
         "incumbent_settings_object_tracking": {
           "__compat": {
             "mdn_url": "https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Promise#Incumbent_settings_object_tracking",
+            "spec_url": "https://html.spec.whatwg.org/multipage/#incumbent-settings-object-tracking-in-promises",
             "description": "Incumbent settings object tracking",
             "support": {
               "chrome": {