docs: fix demos for real this time

Author: Matt Lewis

demos/plunker-assets/plunker-system-config.ejs

@@ -49,6 +49,10 @@ System.config({
     'date-fns': {
       main: './index.js',
       defaultExtension: 'js'
+    },
+    'ngx-contextmenu': {
+      main: './lib/index.js',
+      defaultExtension: 'js'
     }
   }
 });