Arducopter: modify Build.md #865

Job	Run time
build-gcc-heli	2m 18s
build (base)	3m 0s
build (clang)	2m 19s
autotest-heli (sitltest-heli)	12m 23s
autotest (sitltest-copter-tests1a)	12m 5s
autotest (sitltest-copter-tests1b)	13m 45s
autotest (sitltest-copter-tests1c)	12m 48s
autotest (sitltest-copter-tests1d)	12m 28s
autotest (sitltest-copter-tests1e)	14m 4s
autotest (sitltest-copter-tests2a)	13m 57s
autotest (sitltest-copter-tests2b)	25m 42s
	2h 4m 49s

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