Fix vm.overcommit faq

The "Understanding Virtual Memory" article cited when motivating the setting for overcommit_memory had the meaning of
the values 1 and 2 reversed. I found it while reading this comment http://superuser.com/a/200504.

With this commit, I'm trying to make this known to the Redis FAQ reader. The proc(5) man page has it pretty clear:

       /proc/sys/vm/overcommit_memory
              This file contains the kernel virtual memory accounting mode.
              Values are:

                     0: heuristic overcommit (this is the default)
                     1: always overcommit, never check
                     2: always check, never overcommit

              In mode 0, calls of mmap(2) with MAP_NORESERVE are not
              checked, and the default check is very weak, leading to the
              risk of getting a process "OOM-killed".  Under Linux 2.4 any
              nonzero value implies mode 1.  In mode 2 (available since
              Linux 2.6), the total virtual address space on the system is
              limited to (SS + RAM*(r/100)), where SS is the size of the
              swap space, and RAM is the size of the physical memory, and r
              is the contents of the file /proc/sys/vm/overcommit_ratio.