Update pre-installation.rst

source/getting-started/pre-installation.rst

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 Pre Installation
 *****************
 
-  Start with a fresh install of the Operating system of your choice.  Install **only** the operating system at this point as a clean slate is needed for an optimal install.  Please note the recomended OS is Debian Jessie.
+  Start with a fresh install of Debian 8.  Install **only** the operating system at this point as a clean slate is needed for an optimal install.  Please note the recomended OS is Debian Jessie.
   
    **Debian 7+** No other dependencies needed