[XR] Remove extraneous comma from manifest

Packages/manifest.json

   "dependencies": {
     "com.unity.xr.arcore": "1.0.0-preview.20",
     "com.unity.xr.arfoundation": "1.0.0-preview.19",
-    "com.unity.xr.arkit": "1.0.0-preview.16",
+    "com.unity.xr.arkit": "1.0.0-preview.16"
   },
   "registry": "https://staging-packages.unity.com"
 }