Add missing exercise in the assignment section

[oldyellowshed]

Because

Adds a missing exercise that exists in the GitHub CSS exercises repository but isn't listed in the Assignment section of the 'Intro to CSS' page on The Odin Project site.

This PR

• Added 06-cascade-fix

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[MaoShizhong]

Thanks for the PR. It's missing from that list because it's assigned in the next lesson on The Cascade.