Update areq.ngdoc

Request Type: docs

How to reproduce: 

Component(s): 

Impact: small

Complexity: small

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**Detailed Description:**

I faced an issue and didn't find any help in the doc.

**Other Comments:**

Added a way to solve a common mistake that will bring people to the ng:areq page.  When you forget about the app name in the ng-app directive, this error is thrown.