`magit-cherry-mode-map': add

Commit 8618430b added a reference to MAPVAR `magit-cherry-mode-map'
in `magit-cherry-mode's docstring, but the former didn't exist.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #939 from praet/cherry-mode-map

`magit-cherry-mode-map': add