Commit Messages: Fix punctuation

[Ghost-from-V01d]

Because

This pull request fixes inconsistencies in punctuation within the commit message guidelines.
Correcting these minor punctuation issues makes the guidelines clearer and more standardized.

This PR

• Add two missing periods to maintain consistent punctuation in the lesson text.
• Clarify the guideline that subject lines in commit messages should not end with a period.

Issue

Closes #XXXXX

Additional Information

N/A

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

[MaoShizhong]

Thanks for cleaning this up.