samples: mgmt: hawkbit: make references to "frdm_k64f" consistent

The "frdm_k64f" is referred as frdm_k64f and ``frdm_k64f`` in
places, update them to ":ref:`Freedom-K64F <frdm_k64f>`" or
``frdm_k64f``.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>