docs(tutorial/2 - Angular Templates): Running unit test without Firefox installed throws error

I am following the tutorial on AngularJS website. However I got an error when running the provided unit test without having Firefox browser installed on my machine.

Console output:
```
INFO [karma]: Karma v0.12.37 server started at http://localhost:9876/
INFO [launcher]: Starting browser Chrome
INFO [launcher]: Starting browser Firefox
ERROR [launcher]: Cannot start Firefox
	Can not find the binary /Applications/Firefox.app/Contents/MacOS/firefox-bin
	Please set env variable FIREFOX_BIN
INFO [Chrome 46.0.2490 (Mac OS X 10.11.0)]: Connected on socket zZ0c0QemlSHXzVaqVGqC with id 90305753
``` 

I guess it will be much clearer if we also pointed out these browsers requirement on the tutorial documentation.

Author: risan

docs/content/tutorial/step_02.ngdoc

@@ -195,8 +195,15 @@ to ensure that Karma and its necessary plugins are installed. You can do this by
 
 To run the tests, and then watch the files for changes: `npm test`.
 
-* Karma will start a new instance of Chrome browser automatically. Just ignore it and let it run in
-   the background. Karma will use this browser for test execution.
+* Karma will start new instances of Chrome and Firefox browser automatically. Just ignore them and let them run in
+   the background. Karma will use these browsers for test execution.
+* If you only have Chrome browser installed on your machine, make sure to update the karma configuration file before 
+   running the test. Update `unit/test/karma.conf.js` file to only include Chrome browser:
+   
+  <pre>
+    browsers: ['Chrome']
+  </pre>
+  
 * You should see the following or similar output in the terminal:
 
   <pre>