GCS_MAVLink: correct a debugging message #37965

Job	Run time
build (base)	9m 4s
build-gcc-heli	4m 12s
build (clang)	4m 14s
autotest-heli (sitltest-heli)	10m 9s
autotest (sitltest-copter-tests1a)	7m 54s
autotest (sitltest-copter-tests1b)	9m 7s
autotest (sitltest-copter-tests1c)	10m 38s
autotest (sitltest-copter-tests1d)	8m 1s
autotest (sitltest-copter-tests1e)	8m 31s
autotest (sitltest-copter-tests2a)	9m 25s
autotest (sitltest-copter-tests2b)	27m 0s
	1h 48m 15s

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