Fix case of Xfce in appdata description

thunar.appdata.xml.in

@@ -7,7 +7,7 @@
   <summary>File Browser</summary>
   <description>
     <_p>
-      Thunar is a file browser specifically designed for the XFCE Desktop, but also can
+      Thunar is a file browser specifically designed for the Xfce Desktop, but also can
       serve as an alternate file browser for other Desktop environments. It has a simple,
       clean two-pane design for browsing all your files.
     </_p>