media: ext-ctrls-codec-stateless.rst: change a FWHT flag description

Instead of using "- 1", use "minus one".

The reason is that, if the table is split like:

  V4L2_FWHT_FL_COMPONENTS_NUM_MSK  0x00070000  The number of color components
                                               - 1.

One might not realize the meaning of the second line.

As a side effect, using text instead of arabic numbers is
more appropriate for formal documents like this spec.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>