Project: HashMap: Fix hash map lint errors

[goran1010]

Because

This lesson had a missing heading for a note box, and completely wrong indentation levels throughout.

This PR

• Add a level 4 heading to a note-box
• Remove unnecessary indentation of non-list content
• Change the indentation of ordered list items to the correct 3 spaces, to make sure the level 4 heading for the note-box can render properly

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]

Thanks @goran1010. Can the same fixes be applied to the Ruby version of this project as well please?