Documentation: driver-api: fix device_connection.rst kernel-doc error

Using incorrect :functions: syntax (extra space) causes an odd kernel-doc
warning, so fix that.

Documentation/driver-api/device_connection.rst:42: ERROR: Error in "kernel-doc" directive:

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>