docs: fix grammar in Code of Conduct section ('them' → 'it') (#80181)
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## What?
Fixed a grammatical issue in the Code of Conduct section of
`packages/next/README.md`.
Changed “adhere to them” to “adhere to it”.
## Why?
"Code of Conduct" is a singular noun, so using "them" is grammatically
incorrect. The correct pronoun is "it".
## How?
Edited the relevant sentence in `packages/next/README.md`.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>