Add missing shortcut in documentation

MOD+Backspace also triggers BACK.

Author: rom1v

doc/shortcuts.md

@@ -29,7 +29,7 @@ _<kbd>[Super]</kbd> is typically the <kbd>Windows</kbd> or <kbd>Cmd</kbd> key._
  | Resize window to 1:1 (pixel-perfect)        | <kbd>MOD</kbd>+<kbd>g</kbd>
  | Resize window to remove black borders       | <kbd>MOD</kbd>+<kbd>w</kbd> \| _Double-left-click¹_
  | Click on `HOME`                             | <kbd>MOD</kbd>+<kbd>h</kbd> \| _Middle-click_
- | Click on `BACK`                             | <kbd>MOD</kbd>+<kbd>b</kbd> \| _Right-click²_
+ | Click on `BACK`                             | <kbd>MOD</kbd>+<kbd>b</kbd> \| <kbd>MOD</kbd>+<kbd>Backspace</kbd> \| _Right-click²_
  | Click on `APP_SWITCH`                       | <kbd>MOD</kbd>+<kbd>s</kbd> \| _4th-click³_
  | Click on `MENU` (unlock screen)⁴            | <kbd>MOD</kbd>+<kbd>m</kbd>
  | Click on `VOLUME_UP`                        | <kbd>MOD</kbd>+<kbd>↑</kbd> _(up)_