message: use ARIA roles for Dropdown example
diff: ## Default example
  ```.jsx
  <Dropdown scheme="primary" minWidth="5em">
  - <Box is="ul" m={0} p={0} className="list-style-none">
  - <li>Item 1</li>
  - <li>Item 2</li>
  - <li>Item 3</li>
  + <Box role="list">
  + <Box role="listitem">Item 1</Box>
  + <Box role="listitem">Item 2</Box>
  + <Box role="listitem">Item 3</Box>
  </Box>
  </Dropdown>
  ```
  