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

chrome/test/data/extensions/api_test/tts/speak_once/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.
 
 // browser_tests.exe --gtest_filter="TtsApiTest.*"
 
 chrome.test.runTests([
   function testSpeakOnce() {
     function eventListener(event) {
       chrome.test.assertEq('end', event.type);
       chrome.test.assertEq(11, event.charIndex);
       chrome.test.succeed();
     }
     chrome.experimental.tts.speak(
         'hello world',
-        {'onevent': eventListener},
+        {'onEvent': eventListener},
         function() {
           chrome.test.assertNoLastError();
         });
   }
 ]);