docs: Add missing useMDXComponents import (#2370)

Author: Nischal2015

packages/docs/src/pages/mdx/styling.mdx

@@ -14,7 +14,7 @@ of MDX pages, or at the root level of your project, such as in `pages/_app.js`
 for Next.js.
 
 ```jsx
-import { MDXProvider } from '@mdx-js/react'
+import { MDXProvider, useMDXComponents } from '@mdx-js/react'
 import { useThemedStylesWithMdx } from '@theme-ui/mdx'
 import { ThemeProvider } from 'theme-ui'