docs: fix missing TextField import (#8688)

Fixes a missing import in field prop example in
docs/beta/admin/fields.mdx.

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docs/admin/fields.mdx

@@ -217,6 +217,7 @@ Client Component:
 'use client'
 import React from 'react'
 import type { TextFieldClientComponent } from 'payload'
+import { TextField } from '@payloadcms/ui'
 
 export const MyTextField: TextFieldClientComponent = ({ field }) => {
   return <TextField field={field} />