Fix documentation for ns3::BulkSendHelper class reference

The former documentation stated that a typical value for the BulkSendHelper constructor is ns3::UdpSocketFactory, but the UDP protocol is not supported by the BulkSend Application as stated in its documentation 1. This is a factual error in the documentation and particularly confusing for beginners, who may implement a traffic model based on this class only to end up with this error:

Using BulkSend with an incopatible socket type. BulkSend requires SOCK_STREAM or SOCK_SEQPACKET. In other words, use TCP instead of UDP

This commit changes the documentation by stating that a typical value should be ns3::TcpSocketFactory, which is also in compliance with the examples provided in the module.

This issue was originally noted by Chiara Cervelli, Rebecca Di Castro, Francesco Brigante, Alessio Calcagni, and Sara Fabrizi, bachelor students from Sapienza, University of Rome.