message: add more padding around sticky position example
diff: <ExampleHeading my={2}>Sticky</ExampleHeading>
  <Box border={1} borderColor="green.5" height={1000}>
  - <Sticky top={0} bg="green.2" p={4}>
  + <Sticky top={0} bg="green.2" p={6}>
  I'm sticky!
  </Sticky>
  </Box>
  