update broken network programming link

README.md

@@ -216,7 +216,7 @@
 
 #### Build your own `Network Stack`
 
-* [**C**: _Beej's Guide to Network Programming_](http://beej.us/guide/bgnet/html/multi/index.html)
+* [**C**: _Beej's Guide to Network Programming_](http://beej.us/guide/bgnet/)
 * [**C**: _Let's code a TCP/IP stack_](http://www.saminiir.com/lets-code-tcp-ip-stack-1-ethernet-arp/)
 * [**Ruby**: _How to build a network stack in Ruby_](https://medium.com/geckoboard-under-the-hood/how-to-build-a-network-stack-in-ruby-f73aeb1b661b)