Build: Fix effect demo url (typo)

Author: rxaviers

build/core.json

@@ -30,7 +30,7 @@
 		],
 		"category": "effect",
 		"homepage": "http://jqueryui.com/",
-		"demo": "http://jqueryui.com/effects/",
+		"demo": "http://jqueryui.com/effect/",
 		"docs": "http://api.jqueryui.com/category/effects-core/"
 	},
 	"position": {