Make the last option's purpose more explicit

I mistook it several times for the option to exit the menu :/

SDL/gui.c

@@ -239,7 +239,7 @@ static const struct menu_item paused_menu[] = {
     {"Keyboard", enter_controls_menu},
     {"Joypad", enter_joypad_menu},
     {"Help", item_help},
-    {"Exit", item_exit},
+    {"Quit SameBoy", item_exit},
     {NULL,}
 };