docs: fix error(typo) on `with-nextjs page` example code

[taehyunkim3]

• add missing 'function' on example code.

Updating existing pages ✍️

• Add missing 'function' on example code.

• https://swr.vercel.app/docs/with-nextjs

• BEFORE

  'use client'
  import useSWR from 'swr'
  export default Page() {  //👈missing 'function' here
  const { data } = useSWR('/api/user', fetcher)
  return <h1>{data.name}</h1>
  }

• AFTER

  'use client'
  import useSWR from 'swr'
  export default function Page() {
  const { data } = useSWR('/api/user', fetcher)
  return <h1>{data.name}</h1>
  }

Description

• [ ] Adding new page
• [x] Updating existing documentation
• [ ] Other updates

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