Spelling & Grammar updates

[TheJambo]

⚠️⚠️⚠️ Since we do not accept all types of pull requests and do not want to waste your time. Please be sure that you have read pull request rules:
https://github.com/louislam/uptime-kuma/blob/master/CONTRIBUTING.md#can-i-create-a-pull-request-for-uptime-kuma

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• [✔ ] I have read and understand the pull request rules.

Description

Corrects the spelling and grammar of the main readme file and other markdown files.

Type of change

Cosmetic, spelling and grammar fixes of Markdown files.

Please delete any options that are not relevant.

• Other

Checklist

• [✔] My code follows the style guidelines of this project
• [✔] I ran ESLint and other linters for modified files
• [✔] I have performed a self-review of my own code and tested it
• [✔] I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• [✔] My changes generate no new warnings
• [✔] My code needed automated testing. I have added them (this is optional task)

The above aren't really relevant here, but y'know ☺. I also did read that you'd like the PRs to be draft, but I think this one makes more sense to skip straight to a normal review.

Screenshots (if any)

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