[image-guide] fix bash syntax error in a sample rc.local script

Add "fi" to the end of the "if" cycle in a sample rc.local script
to avoid bash syntax error.

Change-Id: I3fa83236f162ceadba724543f26d9058726a3582
Closes-Bug: #1474793

doc/image-guide/section_centos-example.xml

@@ -261,6 +261,7 @@ while [ ! -f /root/.ssh/authorized_keys ]; do
     echo "*****************"
     cat /root/.ssh/authorized_keys
     echo "*****************"
+  fi
 done</programlisting>
         <note>
             <para>Some VNC clients replace the colon (<literal>:</literal>) with a semicolon