Replace APNG spec URL

Author: caugner

mediatypes/image/png.json

@@ -50,7 +50,7 @@
         "animation": {
           "__compat": {
             "description": "Animation support (APNG)",
-            "spec_url": "https://wiki.mozilla.org/APNG_Specification#Overview",
+            "spec_url": "https://w3c.github.io/png/#apng-frame-based-animation",
             "support": {
               "chrome": {
                 "version_added": "1"