ref-manual: Updated list of supported distros.

Had to make some tweaks.

(From yocto-docs rev: 7200f3d0b75e2ffc8b755c4962b532f399b78885)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-system-requirements.xml

@@ -88,13 +88,13 @@
                 </itemizedlist>
             </note>
             <itemizedlist>
-                <listitem><para>Ubuntu 16.04</para></listitem>
+                <listitem><para>Ubuntu 16.04 (LTS)</para></listitem>
                 <listitem><para>Ubuntu 18.04</para></listitem>
                 <listitem><para>Fedora 28</para></listitem>
-                <listitem><para>Centos 7.x</para></listitem>
+                <listitem><para>CentOS 7.x</para></listitem>
                 <listitem><para>Debian GNU/Linux 8.x (Jessie)</para></listitem>
                 <listitem><para>Debian GNU/Linux 9.x (Stretch)</para></listitem>
-                <listitem><para>openSUSE 42.3</para></listitem>
+                <listitem><para>OpenSUSE 42.3</para></listitem>
             </itemizedlist>
         </para>
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