Alignment: Fix Broken Flexbox Guide Link

[Yachou16]

Because

The CSS-Tricks flexbox guide link was returning a 404 error, preventing students from accessing the resource.

This PR

• Updates the broken CSS-Tricks flexbox guide URL to the current working link
• Changes from css-tricks.com/snippets/css/a-guide-to-flexbox/ to css-tricks.com/snippets/css/complete-guide-to-css-flexbox-layout/

Issue

No existing issue - discovered while working through the lesson.

Additional Information

N/A

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
• 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
• If any lesson files are included in this PR, they follow the Layout Style Guide

[mao-sz]

Thanks for the PR but we already have an open issue for this which has been assigned to someone: #30776
And more changes are needed than just the assignment link