message: fix BranchName example
diff: <ExampleHeading mt={3}>BranchName with Octicon</ExampleHeading>
  <BranchName>
  - <Octicon icon={GitBranch} />
  + <Box mr={2} display='inline-block'><Octicon icon={GitBranch} /></Box>
  a_new_feature_branch
  </BranchName>
  