`git-rebase-mode-map': fix docstring typo fix

Commit af96de0e assumed `git-rebase-mode-map's docstring contained
a typo, but it turns out that it was just worded awkwardly.

But instead of reverting that commit, remove the line entirely,
as the code which used to add the keybinds for git-rebase-{pick,...}
to `git-rebase-mode-map' was removed in commit cb15fbe2...

Signed-off-by: Pieter Praet <pieter@praet.org>