docs(guide/concepts.ngdoc) protocol relative URL

Request Type: bug

How to reproduce: Open the angular docs from https: https://docs.angularjs.org/guide/concepts

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**

When accessing the docs from https, the "Accessing the backend" example fails because it contains a hard coded protocol. By making the URL protocol relative, the example should work over http and https.