gitk: Update selection background colorbar in prefs dialog

The callback function was incorrectly set to update the background
colorbar when updated the selection background. This did not affect the
colors chosen or their use, just their presentation in the preferences
dialog box.

Signed-off-by: Mark Levedahl <mdl123@verizon.net>

gitk

@@ -7004,7 +7004,7 @@ proc doprefs {} {
     grid x $top.hunksepbut $top.hunksep -sticky w
     label $top.selbgsep -padx 40 -relief sunk -background $selectbgcolor
     button $top.selbgbut -text "Select bg" -font optionfont \
-	-command [list choosecolor selectbgcolor 0 $top.bg background setselbg]
+	-command [list choosecolor selectbgcolor 0 $top.selbgsep background setselbg]
     grid x $top.selbgbut $top.selbgsep -sticky w
 
     frame $top.buts