developer-guidelines.sgml: Update copyright year

We touched the Development Guidelines this year.

doc/developer-guidelines.sgml

@@ -11,7 +11,7 @@
       </author>
     </authorgroup>
 
-    <pubdate>2009</pubdate>
+    <pubdate>2013</pubdate>
     <title>Nagios plug-in development guidelines</title>
 	
     <revhistory>
@@ -22,7 +22,7 @@
     </revhistory>
 
 	<copyright>
-		<year>2000 - 2009</year>
+		<year>2000 - 2013</year>
 		<holder>Nagios Plugins Development Team</holder>
 	</copyright>
 
@@ -34,7 +34,7 @@
     the plug-in developers and encourage the standarization of the
     different kind of plug-ins: C, shell, perl, python, etc.</para>
 
-        <para>Nagios Plug-in Development Guidelines Copyright (C) 2000-2009
+        <para>Nagios Plug-in Development Guidelines Copyright (C) 2000-2013
         (Nagios Plugins Team)</para>
 
         <para>Permission is granted to make and distribute verbatim