Update theme-ui.md

The link to Typography.js on the Gatsby Theme-UI documentation page was leading to a 404 error. This commit updates the link to point to the correct URL: [new-link](https://www.gatsbyjs.com/docs/using-typography-js/).

Closes gatsbyjs#39034 

- Updated the link in `docs/theme-ui.md`
- Verified that the new link works correctly

This resolves the broken link issue mentioned in issue gatsbyjs#39034 .

docs/docs/how-to/styling/theme-ui.md

@@ -6,7 +6,7 @@ title: Theme UI
 It allows you to style any component in your application with typographic, color, and layout values defined in a shared theme object.
 Theme UI is currently used in Gatsby's official themes,
 but it can be used in any Gatsby site or React application.
-It includes the [Emotion][] CSS-in-JS library along with additional utilities for styling [MDX][] and using configurations and themes from [Typography.js][].
+It includes the [Emotion][] CSS-in-JS library along with additional utilities for styling [MDX][] and using configurations and themes from [Typography.js](https://www.gatsbyjs.com/docs/using-typography-js/).
 
 ## Using Theme UI in Gatsby