This pull request makes minor documentation improvements to the
`robots.mdx` API reference, correcting a typo and clarifying the
description of `robots.js` route handler behavior.

- Documentation clarity and typo fixes:
* Corrected "Route Handlers" to "Route Handler" in the description of
`robots.js` and updated the explanation for clarity.
* Fixed the spelling of "customise" to "customize" for consistency with
American English.* Fixed spelling on some words of the robots.mdx
documentation:
- Handlers (plural) to Handler (singular)
- customise = customize

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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>