[Extension Docs] Update Image Name

Image in notification was removed, but the name of the image
in notification was not updated.

Bug: None
Change-Id: I121a86bf592601933d1f4ae0730e5c049b743dd4
Reviewed-on: https://chromium-review.googlesource.com/769655Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517641}

chrome/common/extensions/docs/examples/api/water_alarm_notification/background.js

@@ -7,7 +7,7 @@ chrome.alarms.onAlarm.addListener(function() {
   chrome.browserAction.setBadgeText({text: ''});
   chrome.notifications.create({
       type:     'basic',
-      iconUrl:  'drink_water.png',
+      iconUrl:  'stay_hydrated.png',
       title:    'Time to Hydrate',
       message:  'Everyday I\'m Guzzlin\'!',
       buttons: [