Remove Ubuntu specification

The linked page is general to linux, and specifically mentions CentOS, Fedora, RHEL and Ubuntu. It does not contain any information that is specific to Ubuntu only.

Author: asmacdo

docs/starting/installation.rst

@@ -18,4 +18,4 @@ for development purposes, as well as setuptools, pip, and virtualenv setup.
 
 - :ref:`Mac OS X <install-osx>`.
 - :ref:`Microsoft Windows <install-windows>`.
-- :ref:`Ubuntu Linux <install-linux>`.
+- :ref:`Linux <install-linux>`.