docs: pm: Fix doxygen tag

Uses tag "option" for CONFIG_PM_DEVICE_RUNTIME to make it
linkable. Just like is done for all others build options.

Signed-off-by: Flavio Ceolin <[email protected]>

Author: Flavio Ceolin

doc/reference/power_management/index.rst

@@ -477,7 +477,7 @@ the following configuration flags.
    This flag is enabled if the SOC interface and the devices support device power
    management.
 
-:code:`CONFIG_PM_DEVICE_RUNTIME`
+:option:`CONFIG_PM_DEVICE_RUNTIME`
 
    This flag enables the Runtime Power Management.