docs(tutorial/step_12): fix missing URL for Bower

Request Type: docs

How to reproduce: 

1. Go to https://docs.angularjs.org/tutorial/step_12

2. Scroll down to the Dependencies section

3. Read the second paragraph

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

A `[Bower][bower]` link is used without a URL definition so an `<a>` tag is never generated.

**Other Comments:**