Update flexbox-growing-and-shrinking.md

[5TQ6]

This video really helped me understand flexbox, so if you can put it in the additional resources so it helps people.

Complete the following REQUIRED checkboxes:

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] 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

Complete the following checkboxes ONLY IF they are applicable to your PR. You can complete them later if they are not currently applicable:

• [x] I have previewed all lesson files included in this PR with the Markdown preview tool to ensure the Markdown content is formatted correctly
• [ ] I have ensured all lesson files included in this PR follow the Layout Style Guide

---

1. Because:

Closes #XXXXX

2. This PR:

3. Additional Information: