Flexbox Alignment: Update links to CSS Tricks 'Guide to Flexbox'

[martupeluso]

Because

The URL for CSS Tricks' "Guide to Flexbox" article changed, so the links in the curriculum had to be updated.

This PR

Updates the links to CSS Tricks' "Guide to Flexbox" in the Flexbox Alignment lesson.

Issue

Closes #30776

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