[css-image-animation] Revert to old explainer URL

The new one is checked into the repo, but so far failing to build. We
can switch back once the build server succeeds.

Author: frivoal

css-image-animation-1/Overview.bs

@@ -20,7 +20,7 @@ Abstract: This CSS module proposes facilities to control the rendering of animat
 		  "Florian Rivoal",
 		  "Lea Verou"
 		],
-		"href": "https://drafts.csswg.org/css-image-animation-1/explainer"
+		"href": "https://github.com/webplatformco/project-image-animation/blob/main/image-animation-property/README.md",
 		"title": "CSS Image Animation Explainer"
 	  },
 	"IMAGE-ANIMATION-TALK": {