Change `` ` `` into `'`

Change `` ` `` into `'`

This is both for the `` ` `` shortcut (go to mark) and the `` ` `` mark
(last position). Both `` ` `` and `'` are used in Vim. `'` is a better
default, because it is easier to type -- both on an en-US QWERTY
keyboard and, more importantly, on some international layouts, such as
the sv-SE layout.

Refs. #788.