config.in: use swaynag -B instead of -b

'-b' spawns a terminal, which is unnecessary for this use case

config.in

@@ -82,7 +82,7 @@ output * bg @datadir@/backgrounds/sway/Sway_Wallpaper_Blue_1920x1080.png fill
     bindsym $mod+Shift+c reload
 
     # Exit sway (logs you out of your Wayland session)
-    bindsym $mod+Shift+e exec swaynag -t warning -m 'You pressed the exit shortcut. Do you really want to exit sway? This will end your Wayland session.' -b 'Yes, exit sway' 'swaymsg exit'
+    bindsym $mod+Shift+e exec swaynag -t warning -m 'You pressed the exit shortcut. Do you really want to exit sway? This will end your Wayland session.' -B 'Yes, exit sway' 'swaymsg exit'
 #
 # Moving around:
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