Fixed mock-api readme.md

[dafreaki]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

fixed typo in readme as per #133 (comment)

Description of Changes:

• fixed grant_type example for POST/token
• fixed spelling for separated

Checklist

• [X ] My code follows the Contribution Guide
• [ X] I have self-reviewed the changes being requested
• [X ] I have made comments on pieces of code that may be difficult to understand for other editors
• [X ] I have updated the documentation (if applicable)