Update documentation links printed in warning output

[granolocks]

I observed a warning like the following:

==== LCM Warning ===                                                                                         
LCM detected that large packets are being received, but the kernel UDP                               
receive buffer is very small.  The possibility of dropping packets due to                             
insufficient buffer space is very high.                                                                                                              
                                                                                                        
For more information, visit:                                                                            
   http://lcm-proj.github.io/multicast_setup.html  

When I followed the provided link I observed that the documentation page URL scheme had changed so I believe this points to the correct parts of the github.io page.

[ihilt]

Looks good to me, thanks!