docs(tutorial/6 - Two-way Data Binding): Fixed Experiments Documentation

In the experiments section, it is suggested to add a `-` symbol to `<option value="age">Oldest</option>`
This change is made to reverse the sort order when selecting the "age" option.

However, this change affects the default $ctrl.orderProp that we had set in app/phone-list/phone-list.component.js
After making the change, our default when refreshing the page is "Sort by: [blank]"

I added some additional documentation to clarify that this behavior makes sense and that the reader should try and fix this within phone-list.component.js

Let me know if this makes sense or if there's anything I can do to help improve the fix.
Thank you!

Author: ravefalcon92

docs/content/tutorial/step_06.ngdoc

@@ -230,6 +230,9 @@ You can now rerun `npm run protractor` to see the tests run.
 
 * Reverse the sort order by adding a `-` symbol before the sorting value:
   `<option value="-age">Oldest</option>`
+  Upon refreshing the page, you'll see that the default selection of the "age" option in "Sort by" is blank again.
+  Try fixing this in **`app/phone-list/phone-list.component.js`:**
+  
 
 
 ## Summary