Update branch names in Developers Guide

docbook/Developers_Guide/en-US/Contributors.xml

@@ -187,9 +187,9 @@ git remote add --tags upstream git://github.com/mantisbt/mantisbt.git
 			</para>
 			<para>
 				The following command will set up a tracking branch for the
-				current stable branch, <literal>master-1.2.x</literal>.
+				current stable branch, <literal>master-1.3.x</literal>.
 <programlisting>
-git checkout -b master-1.2.x origin/master-1.2.x
+git checkout -b master-1.3.x origin/master-1.3.x
 </programlisting>
 			</para>
 
@@ -270,7 +270,7 @@ sed -rn "s/^.*path\s*=\s*(.*)$/\1/p" .gitmodules |xargs -n 1 basename |xargs -I{
 			In order to keep your local repository up-to-date with the
 			official one, there are a few simple commands needed for any
 			tracking branches that you may have, including
-			<literal>master</literal> and <literal>master-1.2.x</literal>.
+			<literal>master</literal> and <literal>master-1.3.x</literal>.
 		</para>
 
 		<para>