thanks Wolfgang Keller

README

@@ -45,6 +45,6 @@ BUILDING AND RUNNING XV6
 To build xv6 on an x86 ELF machine (like Linux or FreeBSD), run
 "make". On non-x86 or non-ELF machines (like OS X, even on x86), you
 will need to install a cross-compiler gcc suite capable of producing
-x86 ELF binaries. See http://pdos.csail.mit.edu/6.828/2016/tools.html.
+x86 ELF binaries. See https://pdos.csail.mit.edu/6.828/2018/tools.html.
 Then run "make TOOLPREFIX=i386-jos-elf-". Now install the QEMU PC
 simulator and run "make qemu".