mavsdk_tests: fix build instructions
mavsdk_tests: fix build instructions

PX4/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware