Fix typos in Creating Diagrams

[bgrzesiak-fetch]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams

What part(s) of the article would you like to see updated?

Just some grammatical typos

Additional information

No response

[welcome]

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[nguyenalex836]

@bgrzesiak-fetch Thank you for opening an issue and linking it to your PR! ✨