`magit-diffstat-keymap': fix docstring

Commit 8618430b changed `magit-diffstat-keymap's docstring to refer to
`magit-diffstat-mode', but that doesn't exist.  `magit-diffstat-keymap'
is used in `magit-commit-mode' and `magit-diff-mode' though.

Signed-off-by: Pieter Praet <pieter@praet.org>