Fix typo (angular#214)

Author: joehalliwell

docs/5-user-authentication.md

@@ -28,7 +28,7 @@ bootstrap(<MyApp>Component, [
 ]);
 ```
 
-**Example boostrap**
+**Example bootstrap**
 ```ts
 import { bootstrap } from '@angular/platform-browser-dynamic';
 import { enableProdMode } from '@angular/core';
@@ -153,4 +153,4 @@ export class RcTestAppComponent {
     });    
   }
 }
-```
+```