docs: driver-model: remove a duplicated markup at driver.rst

The "::" markup is there twice, causing a warning:

    Documentation/driver-api/driver-model/driver.rst:233: WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/52c65b4ebca1262f80d8939e2027358f9b476f00.1592895969.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>