Update tutorial-14.md

I found (format "%.2f" (calculate ...)) was choking on BigInt and Ratio types.  Casting to a double or float fixes the problem.

(Not sure if there's a more elegant solution.  Perhaps it's better to do the casting inside the remote/calculate definition so that it's self contained?)
Merge pull request #95 from olivergeorge/patch-4

Update tutorial-14.md