fix typo (#238)

Author: facuperezm

exercises/01.managing-ui-state/README.mdx

@@ -13,7 +13,7 @@ how that works with React:
 There's a cycle of user interaction, state changes, and re-rendering. This
 is the core of how React works for interactive applications.
 
-The `render` phase is what what we've done so far with creating React elements.
+The `render` phase is what we've done so far with creating React elements.
 Handling user interactions is what we do with event listeners like `onChange`.
 Now we're going to get into the `state changes` bit.