doc: frdm_k64f: Update firmware name for board frdm_k64f

Update the name of the firmware in the board documentation.
Proper name is "Firmware FRDM K64F" based on their website doc.

Change-Id: I75be2a676a9f2dcb4806ffbb39483c1ad91dae76
Signed-off-by: Javier B Perez Hernandez <javier.b.perez.hernandez@linux.intel.com>