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>

Documentation/power/runtime_pm.txt

@@ -506,7 +506,7 @@ pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
 they will fail returning -EAGAIN, because the device's usage counter is
 incremented by the core before executing ->probe() and ->remove().  Still, it
 may be desirable to suspend the device as soon as ->probe() or ->remove() has
-finished, so the PM core uses pm_runtime_idle_sync() to invoke the
+finished, so the PM core uses pm_runtime_put_sync() to invoke the
 subsystem-level idle callback for the device at that time.
 
 The user space can effectively disallow the driver of the device to power manage