doc: nrf: bluetooth_services: fast_pair: Update Kconfig for sysbuild

Updates usage of a Kconfig for sysbuild

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

doc/nrf/libraries/bluetooth_services/services/fast_pair.rst

@@ -135,7 +135,7 @@ Firmware Revision characteristic
 
 The Fast Pair specification requires enabling GATT Device Information Service and the Firmware Revision characteristic.
 For this reason, the default values of the Kconfig options :kconfig:option:`CONFIG_BT_DIS` and :kconfig:option:`CONFIG_BT_DIS_FW_REV`, respectively, are set to enabled.
-The default value of :kconfig:option:`CONFIG_BT_DIS_FW_REV_STR` is set to :kconfig:option:`CONFIG_MCUBOOT_IMGTOOL_SIGN_VERSION` if :kconfig:option:`CONFIG_BOOTLOADER_MCUBOOT` is enabled.
+The default value of :kconfig:option:`CONFIG_BT_DIS_FW_REV_STR` is set to :kconfig:option:`CONFIG_MCUBOOT_IMGTOOL_SIGN_VERSION` if :kconfig:option:`SB_CONFIG_BOOTLOADER_MCUBOOT` is enabled.
 Otherwise, it is set to ``0.0.0+0``.
 
 This requirement does not apply for the locator tag use case as specified in the `Fast Pair Device Feature Requirements for Locator Tags`_ documentation.