Fix parsability of shipped config file

Double hyphen inside of comment breaks xml.dom.minidom XML parser, which
is considered "not a bug" by mantainer: http://bugs.python.org/issue24197

doc/pamusb.conf

@@ -40,8 +40,8 @@ See http://www.pamusb.org/doc/configuring
 			<user id="scox">
 				<device>MyDevice</device>
 				<option name="quiet">true</option>
-				<agent event="lock">gnome-screensaver-command --lock</agent>
-				<agent event="unlock">gnome-screensaver-command --deactivate</agent>
+				<agent event="lock">gnome-screensaver-command -\-lock</agent>
+				<agent event="unlock">gnome-screensaver-command -\-deactivate</agent>
 			</user>
 
 			Configure user root to authenticate using MyDevice, but update one