Ruby/Event manager project: fix formatting issues

[SumonGFC]

Because

Being adapted from another source, this project contained various inconsistencies with the layout style guide.

This PR

1. Replace all references to "large" or "small" sample data with just "sample data"
2. Fix assignment section formatting so that it is consistent with layout style guide
3. Change the level 3 header "Skipping the header row" to a level 4 header

Issue

Closes #23552

Additional Information

I went ahead and made the change indicated by point 3 in the "This PR" section above. Re-reading @scheals first point in the related issue, I went back and looked at the table of contents of the project and noticed there was a somewhat abrupt section between "Iteration 0" and "Iteration 1", which was this "Skipping the header row" section. Changing this section title to a level 4 header will remove it from the table of contents (i think...), so that the tutorial flows a bit nicer (5 parts, iterations 0 through 4, then a single Assignment section). Feel free to not include this change for whatever reason.

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

[JoshDevHub]

Thank you for contributing. Nice changes 🚀