Introduction to Flexbox: Added slaying the dragon tutorial on flexbox

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Because

For intro to flexbox, I added one more tutorial under additional resources: Slaying the dragon youtube channel's 8 minute flexbox guide. This will introduce the web devs to this channel too, as its content is top notch and he goes straight to the topic in his videos.

This PR

• Added slaying the dragon youtube channel's flexbox tutorial as additional resources.

Issue

N/A

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

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Removed the blank lines.