Instruct students to export the box component (#400)

* Update README.mdx

* Update exercises/06.styling/02.solution.component/README.mdx

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Co-authored-by: Kent C. Dodds <[email protected]>

Author: wargha

exercises/06.styling/02.solution.component/README.mdx

@@ -4,3 +4,5 @@
 
 👨‍💼 Great work with the `<Box />` component. But we can make this experience even
 better by designing our box around the different sizes. Let's keep going!
+
+🚨 Also, don't forget to export your Box component (tests will need it!) `export function Box...`