emacs: Refine scope of notmuch-jump-key face

The intent of the 'notmuch-jump-key' face is to allow users/themes to
differentiate the text of the minibuffer prompt from the keys that are
associated with jump actions.  Commit 5cc106b0 correctly introduced the
'notmuch-jump-key' face for keys, but mistakenly applied it to the
prompt as well.