docs: add StyleSheet import

Author: Joehoel

versioned_docs/version-6.x/elements.md

@@ -136,6 +136,7 @@ For example, you can use this with `headerTransparent` to render a blur view to
 
 ```js
 import { BlurView } from 'expo-blur';
+import { StyleSheet } from 'react-native';
 
 // ...