Fix issue #4 on github.

Issue reported by Martin.

In (term-prinl x y any1 [any2 ... anyn])

x and y should be optional.

The current state of the term-prinl function in PicoLisp expects
x and y as mandatory arguments.

Fix the documentation too at
http://en.wikibooks.org/wiki/Alcor6L/PicoLisp/term#term-prinl