switch to using Ctrl+Shift+U in commands example

seems to be assigned to download manager (at least on linux)

commands/README.md

@@ -6,7 +6,7 @@ This extension includes:
 
 All it does is:
 
-* register a shortcut (Ctrl+Shift+Y) to send a command to the extension (Command+Shift+Y on a Mac).",
+* register a shortcut (Ctrl+Shift+U) to send a command to the extension (Command+Shift+U on a Mac).",
 
 It shows:
 

commands/background.js

@@ -7,7 +7,7 @@
  *    [{
  *       name: "toggle-feature",
  *       description: "Send a 'toggle-feature' event to the extension"
- *       shortcut: "Ctrl+Shift+Y"
+ *       shortcut: "Ctrl+Shift+U"
  *    }]
  */
 chrome.commands.getAll(function(commandsArray) {
@@ -19,8 +19,8 @@ chrome.commands.getAll(function(commandsArray) {
 /**
  * Fired when a registered command is activated using a keyboard shortcut.
  *
- * In this sample extension, there is only one registered command: "Ctrl+Shift+Y".
- * On Mac, this command will automatically be converted to "Command+Shift+Y".
+ * In this sample extension, there is only one registered command: "Ctrl+Shift+U".
+ * On Mac, this command will automatically be converted to "Command+Shift+U".
  */
 chrome.commands.onCommand.addListener(function(command) {
   console.log("onCommand event received for message: ", command);

commands/manifest.json

@@ -1,6 +1,6 @@
 {
   "name": "Sample Commands Extension",
-  "description": "Press Ctrl+Shift+Y to send an event (Command+Shift+Y on a Mac).",
+  "description": "Press Ctrl+Shift+U to send an event (Command+Shift+U on a Mac).",
   "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/commands",
   "manifest_version": 2,
   "version": "1.0",
@@ -15,7 +15,7 @@
   },
   "commands": {
     "toggle-feature": {
-      "suggested_key": { "default": "Ctrl+Shift+Y" },
+      "suggested_key": { "default": "Ctrl+Shift+U" },
       "description": "Send a 'toggle-feature' event to the extension"
     }
   }