Fix documentation bug in reduceSeries function

The example given uses the wrong order of parameters in the documentation. 

Attempts to fix #745.

If anyone has ideas about how to make the documentation clearer let me know. I think it might be best to expand on the examples given? 

The example given is a concrete use-case, but perhaps I should add the following notes:

1. map is an alias of mapSeries
2. reduce is an alias of reduceSeries
3. map isn't very useful on its own, it should be used with reduce.