Creating a Grid: Fix grammar and punctuation

[frostBite1418]

Because

A small grammatical fix to make a sentence clearer

This PR

• Add a comma in the sentence

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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