Minor grammar fixes in the developers documentation

[zbraiterman]

What does this PR fix/introduce?

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Checklist

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• [ * ] Docs are successfully building - yarn install && yarn run build.
• [ * ] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• [ * ] All external links have been verified with yarn run check:externals.
• [ * ] My changes follow the Casper docs style guidelines.
• [ * ] All technical procedures have been tested (if you want help with this, mention it in Reviewers).
• [ * ] If structural changes are introduced (not just content changes), cross-browser testing has been completed.

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