Removed a contradiction in the README

I noticed in the README under `An example level` this example:

    solution do
      return false unless repo.status.files.keys.include?("README")
      return false if repo.status.files["README"].untracked
      true
    end

contradicted the note a few lines down:

>**note** Because solution is a Proc, you cannot prematurely return out of it and as a result, must put an explicit return on the last line of the solution block.

So this PR fixes that :+1: