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

Documentation/driver-api/driver-model/driver.rst

@@ -228,8 +228,6 @@ over management of devices from the bootloader, the usage of sync_state() is
 not restricted to that. Use it whenever it makes sense to take an action after
 all the consumers of a device have probed::
 
-::
-
 	int 	(*remove)	(struct device *dev);
 
 remove is called to unbind a driver from a device. This may be