Add 'public' visibility to two variables in MyToken example

[bboston7]

This pull request adds the public visibility to the two variables in the MyToken example that lack any explicit visibility. I have made this change for a couple reasons:

• Consistency. All of the other variables in this example use the public visibility and its omission on these two variables raises questions as to what makes them unique. There is currently no justification in the example for the discrepancy so I believe it is simply an oversight.

• Correctness. I believe it is a bug for currentChallenge to not be public. Without the public visibility, a user cannot easily determine what the current challenge problem they are trying to solve is.

[alexvandesande]

Thanks for your contributions!