Fixed a typo (incorrectly referring to G474RE instead of G431KB)

Author: Tauno Magnusson

targets/TARGET_STM/TARGET_STM32G4/TARGET_STM32G431xB/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if("NUCLEO_G474RE" IN_LIST MBED_TARGET_LABELS)
+if("NUCLEO_G431KB" IN_LIST MBED_TARGET_LABELS)
     add_subdirectory(TARGET_NUCLEO_G431KB)
 endif()