add "z" switch to tar example

Some tar implementations cannot or do not auto detect the compress format.
For example, I saw today:

$ tar xvf kea-0.9.tar.gz
tar: Cannot identify format. Searching...
tar: Cpio file name length 41449 is out of range
tar: Invalid header, starting valid header search.
...
tar: Cpio file name length 39676 is out of range
tar: End of archive volume 1 reached
tar: bcpio vol 1, 0 files, 3811899 bytes read, 0 bytes written in 1 secs (3811899 bytes/sec)
tar: Unexpected EOF on archive file

doc/guide/quickstart.xml

@@ -36,7 +36,7 @@
         <listitem>
           <para>
             Extract the tarball. For example:
-            <screen>$ <userinput>tar xvf kea-&__VERSION__;.tar.gz</userinput></screen>
+            <screen>$ <userinput>tar xvzf kea-&__VERSION__;.tar.gz</userinput></screen>
           </para>
         </listitem>