Docs for flat_map should reference concat_map but instead reference flat_map

Line in question:

`Rx/v2/src/rxcpp/operators/rx-flat_map.hpp`
```
Observables, produced by the CollectionSelector, are merged. There is another operator rxcpp::observable<T,SourceType>::flat_map that works similar but concatenates the observables.
```

Doxygen link:
http://reactivex.io/RxCpp/classrxcpp_1_1observable.html#ab1195b083ffa5a97fb1deb854c1e9bee