message: improve Box example markup
diff: @@ -6,10 +6,11 @@ The Box component serves as a wrapper component for most layout related needs. U
  ```.jsx
  <Box>
  - <Label m={1}>Box can be used to create block level elements & more</Label>
  + Box can be used to create both <Box as="span" bg="green.1">inline</Box> and
  + <Box bg="blue.1">block-level elements,</Box>
  + <Box bg="purple.1" width={[1, 1, 1/2]}>elements with fixed or responsive width and height,</Box>
  + <Box bg="yellow.0" p={4} mt={2}>and more!</Box>
  </Box>
  -
  -<Label m={1}>Default label</Label>
  ```
  ## System props
  