Fix: Typo and lint issues in readme

[usamasulaiman]

What

This PR fixes a typo in the readme. It also adjusts a few heading types based on markdown lint rules mentioned below.

Reference

https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md

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Name	Status	Preview	Comments	Updated (UTC)
remix-validated-form	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 31, 2023 3:33pm