fix trivial format

Request Type: docs

How to reproduce: 

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

While this may be no big deal, I found some issues format style.

"CHANGELOG.md" contain 2-space indent and tab indent.

"CONTRIBUTING.md" contain wasted spaces.

**Other Comments:**

Truth be told, I want to do more great things for ruby, but because my ability clumsy yet, start with these ...!