CONTRIBUTING.md: Mention potential multiple passes of fix script

[MaoShizhong]

Because

Sometimes, fixers that collide in the same or close lines result in Markdownlint only applying some of the fixes. The ignored fixers require running the fix script again to catch and fix. Currently, the curriculum repo contributing guide does not explicitly mention this can happen, even though it's a somewhat common occurrence.

This PR

• Explicitly mentions the possibility of needing multiple passes of a fix script to catch and fix all script-fixable lint errors

Issue

N/A

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