Fix outdated reference to @emotion/core (#1926)

Author: Andarist

site/src/pages/404.js

@@ -1,6 +1,6 @@
 // @flow
 /** @jsx jsx */
-import { jsx } from '@emotion/core'
+import { jsx } from '@emotion/react'
 import Layout from '../layouts'
 
 const NotFoundPage = () => {