Fix media query description in magic-styled-components.mdx. (#403)

website/pages/docs/core-concepts/magic-styled-components.mdx

@@ -40,7 +40,7 @@ Here's how the example above works:
 - `default` is replaced by `theme.transitions.default`
 - `emerald-500` is replaced by `theme.colors['emerald-500']`
 - `#fff` is replaced by `theme.colors['#fff']`, that is not defined, so it uses `#fff` as value
-- `@media (min-width: md)` is replaced by `@media (min-width: ${theme.screens.lg})`
+- `@media (min-width: md)` is replaced by `@media (min-width: ${theme.screens.md})`
 - `lg` is replaced by `theme.fontSizes.lg`
 
 ## Use another component