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.
Fixed up 2 links to use the markup syntax.
Fixed up one link as suggested by @coderoshi
Merge pull request #213 from basho/kr-add-openlimit-to-linuxtuning

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