[boost] Signals documentation error

The documentation for the Signals library shows an Allocator type as the second template parameter to the signal template class when one looks in the Reference section. In the header file and in other references to the usage of the signal template class, no such template parameter exists, and the second parameter is the Combiner. I believe the Allocator type may have been in an early version of the signal template class and the doc was never corrected to take it out.