Update release URL of the CSS2 series

Taking the liberty to fix the URL directly as it's a one character fix.
Next build would have updated it otherwise.

Author: tidoust

index.json

@@ -18691,7 +18691,7 @@
       "currentSpecification": "CSS2",
       "title": "Cascading Style Sheets",
       "shortTitle": "CSS",
-      "releaseUrl": "https://www.w3.org/TR/CSS/",
+      "releaseUrl": "https://www.w3.org/TR/CSS2/",
       "nightlyUrl": "https://drafts.csswg.org/css2/"
     },
     "seriesVersion": "2",