autotest: reduce Copter speed requirement in DO_CHANGE_SPEED

Tools/autotest/ArduCopter_Tests/DO_CHANGE_SPEED/mission.txt

@@ -6,6 +6,6 @@ QGC WPL 110
 4	0	3	16	0.000000	0.000000	0.000000	0.000000	-35.360174	149.167908	14.000000	1
 5	0	0	178	1.000000	10.000000	0.000000	0.000000	0.000000	0.000000	0.000000	1
 6	0	3	16	0.000000	0.000000	0.000000	0.000000	-35.363296	149.169244	18.000000	1
-7	0	0	178	1.000000	17.000000	0.000000	0.000000	0.000000	0.000000	0.000000	1
+7	0	0	178	1.000000	16.000000	0.000000	0.000000	0.000000	0.000000	0.000000	1
 8	0	3	16	0.000000	0.000000	0.000000	0.000000	-35.364115	149.165747	20.000000	1
 9	0	0	20	0.000000	0.000000	0.000000	0.000000	0.000000	0.000000	0.000000	1

Tools/autotest/arducopter.py

@@ -7773,7 +7773,7 @@ class AutoTestCopter(AutoTest):
         self.set_parameter("ANGLE_MAX", 6000)
         self.wait_current_waypoint(6)
         self.wait_groundspeed(
-            16.5, 18.5,
+            15.5, 16.5,
             minimum_duration=10,
             timeout=60,
         )