Shopping Cart: Add point to assignment

[nikitarevenco]

…ation

Because

One of the lessons prior to Shopping Cart touches on react PropTypes - however this point is not included in the assignment

This PR

• Tells the learner that they need to make sure to type check their props

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project 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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