FlightTaskDescned: fix horizontal acceleration overriding vertical one #296

Job	Run time
build (check_format)	1m 0s
build (check_newlines)	1m 25s
build (tests)	5m 32s
build (tests_coverage)	7m 6s
build (px4_fmu-v2_default stack_check)	3m 1s
build (validate_module_configs)	48s
build (shellcheck_all)	1m 27s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 3s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 24s
build (px4_sitl_allyes)	4m 15s
build (airframe_metadata)	53s
build (module_documentation)	57s
build (parameters_metadata)	1m 0s
	34m 51s

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