docs: correct typos and style inconsistencies in why‑plotsquared.md

[VulpineFriend87]

Overview

Typos and style inconsistencies in why-plotsquared.md

Description

• Uniform version range notation (1.13.2–1.21.x)
• Consistent heading capitalization
• Grammar and phrasing improvements
• Cleaned up list item punctuation and overall formatting

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.