docs: fix typos and code examples

[illyakurochkin]

This PR fixes several typos and code examples in documentation

Checklist:

• If this PR contains changes to the library itself (not necessary for e.g. docs updates), please include a changeset (see CONTRIBUTING.md)
• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@illyakurochkin: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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[changeset-bot]

⚠️ No Changeset found

Latest commit: fbb9d93

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[phryneas]

Thanks a ton, these will surely help people a lot! :)