message: use Block in DonutChart examples
diff: @@ -175,7 +175,7 @@ const Index = props => (
  </Block>
  </Example>
  <Example name='DonutChart'>
  - <Box mb={2}>
  + <Block mb={2}>
  <Heading tag='h2' fontSize={3} mb={1}>With <Text mono>data</Text> prop</Heading>
  <DonutChart data={{error: 2, pending: 3, success: 5}} />
  {' '}
  @@ -188,8 +188,8 @@ const Index = props => (
  <DonutChart data={{pending: 1, queued: 1}} />
  {' '}
  <DonutChart data={{unknown: 1}} />
  - </Box>
  - <Box mb={2}>
  + </Block>
  + <Block mb={2}>
  <Heading tag='h2' fontSize={3} mb={1}>With <Text mono>DonutSlice</Text> children</Heading>
  <DonutChart>
  <DonutSlice value={1} state='pending' />
  @@ -221,8 +221,8 @@ const Index = props => (
  <DonutChart>
  <DonutSlice value={1} state='queued' />
  </DonutChart>
  - </Box>
  - <Box mb={2}>
  + </Block>
  + <Block mb={2}>
  <Heading tag='h2' fontSize={3} mb={1}>With custom <Text mono>fill</Text> colors</Heading>
  <DonutChart>
  <DonutSlice value={1} fill={theme.colors.purple[0]} />
  @@ -231,7 +231,7 @@ const Index = props => (
  <DonutSlice value={1} fill={theme.colors.purple[3]} />
  <DonutSlice value={1} fill={theme.colors.purple[4]} />
  </DonutChart>
  - </Box>
  + </Block>
  </Example>
  <Example name='Flash'>
  <Block mb={3}>
  