Update step_02.ngdoc

Request Type: test

How to reproduce: Walk through step 2  on a machine without karma cli. Can't run the bash testing script.

Component(s): misc core

Impact: medium

Complexity: small

This issue is related to: 

**Detailed Description:**

It's just an improvement to make the tutorial actually work if you've never installed karma before

**Other Comments:**

Line 189-Modified line to say what to install. Without -g karma-cli it won't run.