docs('lettable' examples): Fix 'lettable' examples (#221)
docs('lettable' examples): Fix 'lettable' examples (#221)

Update (map, filter) example pipe usage to specify path for expected modules, in both
ES & CommonJS example lines. Previously it was incorrectly importing the pipe module itself rather
than the pipe subdirectory index.js.
Update example usage of `of` to be called on the Iterable object. Previously it was not defined before
being called.

closes issue #212
ReactiveX/IxJS,ReactiveX/IxJS,ReactiveX/IxJS