Use correct session file.

debian/postinst

@@ -17,7 +17,7 @@ fi
 
 if [ "$1" = configure|upgrade ]; then
   if [ -x /usr/lib/lightdm/lightdm-set-defaults ] ; then
-    /usr/lib/lightdm/lightdm-set-defaults --keep-old --session=lubuntu || true
+    /usr/lib/lightdm/lightdm-set-defaults --keep-old --session=Lubuntu || true
     /usr/lib/lightdm/lightdm-set-defaults --keep-old --greeter=lightdm-gtk-greeter || true
   fi
 fi

debian/postrm

@@ -9,7 +9,7 @@ fi
 case "$1" in
   remove|purge)
     if [ -x /usr/lib/lightdm/lightdm-set-defaults ] ; then
-      /usr/lib/lightdm/lightdm-set-defaults --remove --session=lubuntu || true
+      /usr/lib/lightdm/lightdm-set-defaults --remove --session=Lubuntu || true
       /usr/lib/lightdm/lightdm-set-defaults --remove --greeter=lightdm-gtk-greeter || true
     fi
     ;;