docs(CONTRIBUTING.md): anchor fix

Request Type: docs

How to reproduce: 1) Open https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md

2) search for the text "Follow our Coding Rules"

3) Click on the "Coding Rules" anchor

4) It doesn't take you to coding rule section.

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**

"Follow our Coding Rules" should link to "#rules" and not "#coding-rules"