PM: Documentation: fix typo: pm_runtime_idle_sync() doesn't exist.

Replace reference to pm_runtime_idle_sync() in the driver core with
pm_runtime_put_sync() which is used in the code.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>