Linting and RuboCop lesson: Fix weird phrasing

[McHeph]

Because

There is a sentence phrased weirdly: "Don’t forget the it must a dotfile, meaning it needs to have a dot before its name."
This PR slightly changes the phrasing so it's clear.

This PR

-The sentence "Don’t forget the it must a dotfile" has been changed to "Don’t forget that it must be a dotfile"

Issue

Closes #28335

Additional Information

No 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

[McHeph]

I just saw my linter changed some of the markdown, I'm fixing it now