Update TTS extension API docs to reflect latest changes. Note that this

chrome/test/data/extensions/api_test/tts/speak_error/test.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // TTS api test for Chrome on ChromeOS.
 // browser_tests.exe --gtest_filter="TtsApiTest.*"
 
 chrome.test.runTests([
   function testSpeakError() {
     var callbacks = 0;
     chrome.experimental.tts.speak(
         'first try',
         {
          'enqueue': true,
-         'onevent': function(event) {
+         'onEvent': function(event) {
             chrome.test.assertEq('error', event.type);
             chrome.test.assertEq('epic fail', event.errorMessage);
             callbacks++;
          }
         },
         function() {
           chrome.test.assertNoLastError();
         });
     chrome.experimental.tts.speak(
         'second try',
         {
          'enqueue': true,
-         'onevent': function(event) {
+         'onEvent': function(event) {
             chrome.test.assertEq('end', event.type);
             callbacks++;
             if (callbacks == 2) {
               chrome.test.succeed();
             } else {
               chrome.test.fail();
             }
          }
         },
         function() {
           chrome.test.assertNoLastError();
         });
   }
 ]);