docs: correct typo in multi-env title

[RobHannay]

Summary

Sorry, just a small typo I noticed when reading this part of the docs.

Thanks for the great tooling 👌

Related Links

Docs page: https://rsbuild.dev/guide/advanced/environments

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[chenjiahan]

Thanks!