Fix typo in 'Simulating the web api' section.

closes #1655

Author: argubihelmi

public/docs/ts/latest/tutorial/toh-pt6.jade

@@ -80,7 +80,7 @@ code-example(language="bash").
 
 .alert.is-helpful
   :marked
-    This chaper is an introduction to the Angular http client.
+    This chapter is an introduction to the Angular http client.
     Please don't be distracted by the details of this backend substitution. Just follow along with the example.
 
     Learn more later about the *in-memory web api* in the [Http chapter](../guide/server-communication.html#!#in-mem-web-api).