Add missing import to site example

Closes GH-2115.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Co-authored-by: Titus <tituswormer@gmail.com>

Author: elpddev

docs/guides/syntax-highlighting.server.mdx

@@ -153,6 +153,8 @@ to interpret the `meta` field.
 For example, it’s possible to pass that string as a prop with a rehype plugin:
 
 ```js path="rehype-meta-as-attributes.js"
+import {visit} from 'unist-util-visit'
+
 /** @type {import('unified').Plugin<Array<void>, import('hast').Root>} */
 function rehypeMetaAsAttributes() {
   return (tree) => {