Update source/languages/en/riak/cookbooks/Linux-Performance-Tuning.md

* Added an h3 section called "Open Files Limit" right after the h2
  section "Linux Tuning", describing the need of increasing the
  open-files limit.

Explanation: the open-files limit issue should be a part of the tuning description,
because the limit increase is required for operating Riak instances properly,
as well as running the tools such as riak_bench.