[boost] [move] Documentation example typo.

Hi. Just reporting a slight typo in boost::move library documentation's file_descriptor class example code for closing the OS file descriptor. It says:

if(!os_descr_) operating_system_close_file(os_descr_);

in its destructor & assignment operator when it should in fact say:

if(os_descr_) operating_system_close_file(os_descr_);

[notice the removed ! operator] Hope this helps. Best regards, Jurko Gospodnetić