Update the link to the Usenix93 Paper on BPF to a pdf format (#70)

Link on https://www.tcpdump.org/papers/bpf-usenix93.pdf

README.md

@@ -270,4 +270,4 @@ Related Projects<a name="others"></a>
 
 * [Traffic Data Repository at the WIDE Project](https://www2.sonycsl.co.jp/person/kjc/papers/freenix2000/): It becomes increasingly important for both network researchers and operators to know the trend of network traffic and to find anomaly in their network traffic. This paper describes an on-going effort within the WIDE project to collect a set of free tools to build a traffic data repository containing detailed information of our backbone traffic. Traffic traces are collected by tcpdump and, after removing privacy information, the traces are made open to the public. We review the issues on user privacy, and then, the tools used to build the WIDE traffic repository. We will report the current status and findings in the early stage of our IPv6 deployment.
 
-* [Usenix93 Paper on BPF](ftp://ftp.ee.lbl.gov/papers/bpf-usenix93.ps.Z): The libpcap interface supports a filtering mechanism based on the architecture in the BSD packet filter. BPF is described in the 1993 Winter Usenix paper "The BSD Packet Filter: A New Architecture for User-level Packet Capture".
+* [Usenix93 Paper on BPF](https://www.tcpdump.org/papers/bpf-usenix93.pdf): The libpcap interface supports a filtering mechanism based on the architecture in the BSD packet filter. BPF is described in the 1993 Winter Usenix paper "The BSD Packet Filter: A New Architecture for User-level Packet Capture".