docs: matter: light swithc: fixed descriminator change note

Fixed an outdated discriminator update note

Signed-off-by: Adam Maciuga <[email protected]>

Author: Adam-Maciuga

samples/matter/light_switch/README.rst

@@ -349,7 +349,7 @@ After building this and the :ref:`Matter Light Bulb <matter_light_bulb_sample>`
    If both are powered up at the same time, the CHIP Tool can commission a random device and the node ID assignment is also random.
    When one device is commissioned, power up the next device and perform the commissioning.
 
-   To avoid this unclear situation, you can set up your unique discriminator in the :file:`src/chip_project_config.h` file by changing the :kconfig:option:`CHIP_DEVICE_CONFIG_USE_TEST_SETUP_DISCRIMINATOR` value.
+   To avoid this unclear situation, set the value of the :kconfig:option:`CONFIG_CHIP_DEVICE_DISCRIMINATOR` Kconfig option in the :file:`prj.conf` file to adjust your unique discriminator.
    Then build an example and commission with your unique discriminator.
 
 .. matter_light_switch_sample_prepare_to_testing_start