Foundations - Growing and Shrinking: Clarify Assignment Requirement

[zunaid-hassan]

Lesson: https://www.theodinproject.com/lessons/foundations-growing-and-shrinking#assignment

Make the instructions for assignment no. 1 explicitly clear as to which section requires reading.

Because

Make the assignment requirement explicitly clear regarding which sub-section needs to be studied, by writing the name of the sub-section.

This PR

Adds the text "(7.1.1. Basic Values of 'flex')" after the document link.

Issue

Closes #XXXXX

Additional Information

A Discord exchange of a student getting demotivated due to misunderstanding the requirement and thinking they need to read the whole document (17,000 words by their estimation): https://discord.com/channels/505093832157691914/505093832157691916/1421431981647855707

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