Update bower.json for version 12.0.0-alpha2

bower.json

@@ -18,7 +18,7 @@
     "tests"
   ],
   "dependencies": {
-    "vaadin-core": "12.0.0-alpha1",
+    "vaadin-core": "vaadin-core#12.0.0-alpha2",
     "vaadin-board": "vaadin-board#2.1.0-alpha1",
     "vaadin-charts": "vaadin-charts#6.2.0-alpha1",
     "vaadin-confirm-dialog": "vaadin-confirm-dialog#1.1.0-alpha1",