cmd: fdt: Align checksign parameter names in help text

The help text references 'addr' as an optional key start address,
but the explanation references the same as 'start', make sure they
both read as 'addr'. Also update the abbreviated 'addr' in the
explanation to 'address'.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>