documentation patch, add () when instantiation multi_index

Here is an abridged error message that i get if i don't have () in my code.

error: no matching function for call to 'boost::multi_index::multi_index_container
[...]
note: boost::multi_index::multi_index_container<Value, IndexSpecifierList, Allocator>::multi_index_container(const boost::multi_index::multi_index_container<Value, IndexSpecifierList, Allocator>&)
[...]
note:   candidate expects 1 argument, 0 provided