Clarify converting an issue to pull request.

Running the example as is will result in a 422 error saying
there's no commits between master and new-feature. This adds a
note saying that you need a new-feature branch that's ahead of
master to run the example.
Fix that typo.
Backtick those branches.
Make steps explicit.
Merge pull request #226 from rsese/clarify-issue-to-pull-request

Clarify issue to pull request