driver-core: platform: fix typo in documentation for multi-driver helper

The function name in the documentation lacked an 's'. Add it.

Fixes: dbe2256d ("driver-core: platform: Provide helpers for multi-driver modules")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>