keyboard: Fix keyboard shortcut label

The instructions label says:
Press Esc to cancel or Backspace to reset the keyboard shortcut.
but the Backspace key set the shortcut to be empty, and disables the
feature. Reset is achieved clicking the button to the right handside of
the shortcut in the main window, and is only visible when the shortcut
isn't set to the default value.
11 jobs for wip/hadess/fix-shortcut-disable-label in 7 minutes and 5 seconds (queued for 3 seconds)