fixed missing arguments in readme examples

It looked like each 'draw-solid call was missing its 'g2d argument, so I converted each s-exp to anonymous fns so that '->img can thread them properly.

I also changed "mexican-hat" to a more precise/concise name, "sombrero". Technically, "sombrero" just means hat with brim. The type of hat alluded to here is referred to as a "sombrero charro" in Mexico.

If you had those examples there for a reason or you have any feedback on anonymizing those fns like that, please let me know. I'm fairly comfortable on my own in Clojure, but I've pretty much learned in isolation so far and I've still got much more to learn.

By the way, this library is wonderful. I've had a lot of fun with it and I really appreciate the work you've put into it. I would love to offer whatever help I can.