Fix LINQPad sample

[sblom]

Tried running the samples in LINQPad 6.10. This is amazing stuff!

Found one broken that won't compile and run, but the fix is simple. The Do() extension method is expecting an IEnumerable<T>, but the new Rectangle() on line 51 is a single item. I tried to come up with some clever overload or changing behavior based on type checking, but this seemed simpler, and certainly works.