fix: update renamed variable in the tRPC Usage page

[jotaRenan]

✅ Checklist

• I have followed every step in the contributing guide (updated 2022-10-06).
• The PR title follows the convention we established conventional-commit
• I performed a functional test on my final commit

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Changelog

Updated a reference to the variable api contained in the code excerpt that precedes this paragraph of the docs. The variable used to be name trpc before #934 (more specifically, here).

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Name	Status	Preview	Comments	Updated
create-t3-app	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Jan 30, 2023 at 3:44AM (UTC)

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	100
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	93
🟠 PWA	54

Lighthouse ran on https://create-t3-app-git-fork-jotarenan-patch-1-t3-oss.vercel.app/

[c-ehrlich]

Thanks, nice catch :)