docs: add missing closing curly brace to code snippet (#61934)
Closed the missing `}` in the [Theme Detection
Picture](https://nextjs.org/docs/app/api-reference/components/image#theme-detection-picture)
example code.
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Co-authored-by: Steven <steven@ceriously.com>
Co-authored-by: Balázs Orbán <info@balazsorban.com>
