docs: Fix some typos on the Upgrading to Docusaurus v3 page

[dmitriyrotaenko]

Hi, team!

Thank you for the amazing Docusaurus.

While following the guide to upgrade to v3, I spotted a number of non-critical typos.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• [ ] If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• [ ] If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fixed a couple of typos in the text.

Test Plan

N/A

Test links

N/A

[meta-cla]

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[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[slorber]

LGTM thanks 👍