Fix the helloworld example by importing the network.js file (PR 8617 follow-up)

Fixes https://stackoverflow.com/q/45393309/5317012

examples/helloworld/hello.js

@@ -3,11 +3,12 @@
 // In production, the bundled pdf.js shall be used instead of SystemJS.
 Promise.all([System.import('pdfjs/display/api'),
              System.import('pdfjs/display/global'),
+             System.import('pdfjs/display/network'),
              System.resolve('pdfjs/worker_loader')])
        .then(function (modules) {
   var api = modules[0], global = modules[1];
   // In production, change this to point to the built `pdf.worker.js` file.
-  global.PDFJS.workerSrc = modules[2];
+  global.PDFJS.workerSrc = modules[3];
 
   // Fetch the PDF document from the URL using promises.
   api.getDocument('helloworld.pdf').then(function (pdf) {