Fixed .exit-off-canvas not closing 'overlap' menu

Old (buggy) behavior:
---
When the menu open_type was set to 'overlap', the .exit-off-canvas
link had no effect.

Cause
---
The `.offcanvas-overlap .exit-off-canvas` selector was missing. A
selector of `.offcanvas-overlap .exit-offcanvas-menu` was where
the `.offcanvas-overlap .exit-off-canvas` selector should have been

Fix
---
- Fix selector in scss/foundation/components/_offcanvas.scss
- No documentation changes are necessary because it now conforms
  to the documentation.  The string "exit-offcanvas-menu" is not
  found anywhere else in the repository.