Update http to https for ChromeVox Tutorial Link

AX-Relnotes: Update http to https for ChromeVox Tutorial Link (tiny security fix)
Change-Id: Ia0d40e04f7165127d4f0b8e3db4e29cb96c40ded
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387408
Commit-Queue: Josiah Krutz <josiahk@google.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803907}

ui/accessibility/extensions/chromevoxclassic/chromevox/background/background.js

@@ -359,7 +359,7 @@ cvox.ChromeVoxBackground.prototype.addBridgeListener = function() {
       chrome.tabs.create(explorerPage);
       break;
     case 'HelpDocs':
-      var helpPage = {url: 'http://chromevox.com/tutorial/index.html'};
+      var helpPage = {url: 'https://chromevox.com/tutorial/index.html'};
       chrome.tabs.create(helpPage);
       break;
     case 'Options':