minor fix

Author: ramiy

css/at-rules.json

@@ -330,7 +330,7 @@
   },
   "@property": {
     "syntax": "@property <custom-property-name> {\n  <declaration-list>\n}",
-    "status": "experimental"
+    "status": "experimental",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@property"
   },
   "@supports": {