Invalid headers in fusion documentation.

[juraj.ivancic@…]

Fusion documentation for set and map contains invalid headers:

#include <boost/fusion/container/set_fwd.hpp> instead of #include <boost/fusion/container/set/set_fwd.hpp>

and

#include <boost/fusion/container/map_fwd.hpp> instead of #include <boost/fusion/container/map/map_fwd.hpp>

[Steven Watanabe]

(In [62891]) Fix headers for set_fwd and map_fwd. Fixes #3237