docs: import box from styled components (#96)

Author: StevePorter92

website/src/pages/docs/responsive.mdx

@@ -13,7 +13,7 @@ Handling responsive is very easy with `@xstyled/system` either using props or ut
 Using [style props](/docs/style-props/), you can tweak a component right from props. Each one of them accepts an object to define responsive values for each prop.
 
 ```js
-import { Box } from '@xstyled/system'
+import { Box } from '@xstyled/styled-components'
 
 /* The box will be displayed only in mobile! */
 const OnlyMobileBox = () => <Box display={{ sm: 'block', md: 'none' }} />