Update links for css-overscroll-behavior (#6967)

- Fixed link to draft spec
- Add link to MDN

Author: evnbr

features-json/css-overscroll-behavior.json

@@ -1,7 +1,7 @@
 {
   "title":"CSS overscroll-behavior",
   "description":"CSS property to control the behavior when the scroll position of a scroll container reaches the edge of the scrollport.",
-  "spec":"https://w3c.github.io/csswg-drafts/css-overscroll-behavior-1/",
+  "spec":"https://drafts.csswg.org/css-overscroll/#overscroll-behavior-properties",
   "status":"wd",
   "links":[
     {
@@ -16,6 +16,10 @@
       "url":"https://developers.google.com/web/updates/2017/11/overscroll-behavior",
       "title":"Google Developers blog post on overscroll-behavior"
     },
+    {
+      "url": "https://developer.mozilla.org/en-US/docs/Web/CSS/overscroll-behavior",
+      "title": "CSS overscroll-behavior on MDN"
+    },
     {
       "url":"https://bugs.webkit.org/show_bug.cgi?id=176454",
       "title":"WebKit support bug"