Fixed the MLton guide's example for Lazy.

MLton would otherwise leak memory for the original example, since the thunk parameter (`th`)
is unnecessarily held in the returned function's closure after evaluation.

Additionally, if the `th` parameter raises an exception, the returned function only
raises that same exception from that point on, rather than re-running `th .
Merge pull request #38 from DanRosenwasser/master

Fixed the MLton guide's example for Lazy.