(Docs) Fix typos in recipes/working-with-themes.md

[lqze]

Description

A few various typos on this file (most likely due to haste in writing!).
Have found and corrected all typos.

Due to this being a minor fix, I have gone to straight to opening a PR instead of an issue.

Documentation

From the official Gatsby Recipes: Working with themes page.

Related Issues

N/A

[laurieontech]

Thanks so much for catching these!

[gatsbot]

Holy buckets, @lqze — we just merged your PR to Gatsby! 💪💜

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