message: change tag to is in ExampleHeading
diff: import React from 'react'
  import {Heading} from '../../src'
  -const ExampleHeading = props => <Heading tag="h3" fontSize={3} mb={2} {...props} />
  +const ExampleHeading = props => <Heading is="h3" fontSize={3} mb={2} {...props} />
  export default ExampleHeading
  