message: docs: update Heading example with sizes
diff: @@ -5,6 +5,7 @@ description: Headings are used for rendering headlines.
  ---
  import ComponentLinks from "../../../src/components/component-links"
  +import { Box } from "@chakra-ui/core"
  Headings are used for rendering headlines.
  @@ -40,22 +41,22 @@ decrease in size for smaller screens.
  ```jsx
  <Stack spacing={3}>
  <Heading as="h1" size="2xl">
  - In love with React & Next
  + (2xl) In love with React & Next
  </Heading>
  <Heading as="h2" size="xl">
  - In love with React & Next
  + (xl) In love with React & Next
  </Heading>
  <Heading as="h3" size="lg">
  - In love with React & Next
  + (lg) In love with React & Next
  </Heading>
  <Heading as="h4" size="md">
  - In love with React & Next
  + (md) In love with React & Next
  </Heading>
  <Heading as="h5" size="sm">
  - In love with React & Next
  + (sm) In love with React & Next
  </Heading>
  <Heading as="h6" size="xs">
  - In love with React & Next
  + (xs) In love with React & Next
  </Heading>
  </Stack>
  ```
  