Advanced Grid Properties: Add missing factors to grid content width calc

[olgacoz]

Because

Text is missing the paddings, and borders of the grid when calculating it's content width. Also it says the browser has to know how wide our grid is but actually means how wide our grid's content width is.

This PR

• Adds the missing factors into the grid's content width calculation.
• Uses "grid's content width" wording to avoid misunderstanding.

Issue

Closes #30969

Additional Information

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

[mao-sz]

You don't need to close anything - a maintainer will do what's needed when a PR is approved.