Fix some typos in custom scripts docs

[tkindy]

Thanks for making Boop, I'm loving it so far!

I noticed a couple typos in the 'Custom Scripts' docs, so figured I'd help out 😄

[sonarqubecloud[bot]]

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[tkindy]

Ah, I just saw https://github.com/IvanMathy/Boop/pull/274 fixes one of the typos I fixed here too. Totally cool if you wanna close this and incorporate the extra fix into that PR!