Update README.md

Theory/README.md

@@ -4,4 +4,3 @@ This chapter describes various concepts which are used in the Linux kernel.
 
 * [Paging](http://0xax.gitbooks.io/linux-insides/content/Theory/Paging.html)
 * [Elf64 format](http://0xax.gitbooks.io/linux-insides/content/Theory/ELF.html)
-* [Per-CPU variables](http://0xax.gitbooks.io/linux-insides/content/Theory/per-cpu.html)