Changes magic sequence to '!#mojo ' and strips 'mojo ' when building url

examples/js/users-guide.md

 # Running Mojo Applications
 
 A Mojo application written in JavaScript is launched with mojo_shell like this:
 
 ```
   mojo_shell <js-application-url>
 ```
 
 Where js-application-url is a URL understood by the shell. For example
 a file or an http URL that names a JS source file. The JS file itself
 must begin with a Mojo "shebang" that specifies the Mojo URL of the JS
 content handler. In other words, the first line of the JS source file
 must be:
 
 ```javascript
-#!mojo:js_content_handler
+#!mojo mojo:js_content_handler
 ```
 
 Following the shebang should be a single AMD module called "main" whose value
 is an Application subclass. The JS content handler will create an instance of
 the Application and make it the client of the Mojo shell. The JS content handler
 is itself a Mojo application and it's responsible for creating an instance of V8
 and loading the "main" JS module and all of the modules the main module
 depends on. 
 
 ## JavaScript Classes
 
 The JS content handler depends on the ECMAScript6 ("Harmony") classes feature.
 
 As of January 2015 Chrome enables Harmony classes by default.
 
 
 # The JS Application Class
 
 Mojo JS applications are defined with the Application class. The
 Application class handles incoming requests for services and provides
 services of its own.
 
 This is the overall structure of a JS Mojo application:
 
 ```javascript
-#!mojo:js_content_handler
+#!mojo mojo:js_content_handler
 
 define("main", ["mojo/services/public/js/application", 
   <list of other modules that this application depends on>
 ],
     function(application, <one parameter per dependent module>) {
       class MyApplication extends application.Application {
         constructor(appShell, url) {
           super(appShell, url); // Initializes this.shell, this.url.
           // MyApplication initializations here. 
         }
@@ skipping 370 matching lines @@
 ### Requesting a Service Using the Application's Shell
 
 The Shell's `connectToService()` method returns a proxy to a Mojo
 service provided by another application. The proxy can be used immediately.
 
 Given the URL of a Mojo application that implements the EchoService we
 can use the application's shell to get an EchoService proxy. Here's a
 complete application:
 
 ```javascript
-#!mojo:js_content_handler
+#!mojo mojo:js_content_handler
 
 define("main", [
   "console",
   "mojo/services/public/js/application",
   "services/js/test/echo_service.mojom"
 ], function(console, appModule, echoModule) {
 
   class EchoShellRequest extends appModule.Application {
     initialize(args) {
       var url = "file:/foo/bar/echo.js";
@@ skipping 16 matching lines @@
 
 The echo_share.js and echo_share_target.js applications demonstrate this.
 
 
 ### Providing a Service with an Application's ServiceProvider
 
 A complete application that unconditionally provides the EchoService
 looks like this:
 
 ```javascript
-#!mojo:js_content_handler
+#!mojo mojo:js_content_handler
 
 define("main", [
   "mojo/services/public/js/application",
   "services/js/test/echo_service.mojom"
 ], function(appModule, echoModule) {
 
   class EchoService extends appModule.Application {
     acceptConnection(initiatorURL, serviceProvider) {
       function EchoServiceImpl(client) {
         this.echoString = function(s) {
@@ skipping 20 matching lines @@
 
 The echo_share.js and echo_share_target.js applications demonstrate this.
 
 ## Final note
 
 An initiator's serviceProvider object can be retained and used to
 request or provide services at any time, not just from within
 application's acceptConnection() method.