fix(package): update core-js to version 3.3.1

Closes #42

Author: greenkeeper[bot]

demo/package.json

@@ -42,7 +42,7 @@
     "@ng-bootstrap/ng-bootstrap": "^4.1.0",
     "@stackblitz/sdk": "^1.3.0",
     "bootstrap": "^4.3.1",
-    "core-js": "^2.6.5",
+    "core-js": "^3.3.1",
     "font-awesome": "^4.7.0",
     "rxjs": "^6.3.3",
     "zone.js": "^0.8.26"