keymap: fix description of xkb_keymap_key_get_syms_by_level()

The @level argument is restricted by xkb_keymap_num_levels_for_key(). Fix
the description to no longer mention xkb_keymap_num_layouts_for_key().

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
keymap: fix typo

Signed-off-by: Ran Benita <ran234@gmail.com>