Fix the Ch4 override value in the channel_overrides example

[Abhayindia]

Found while running the examples against SITL for the sweep in #1255.

channel_overrides.py:74 prints "Set Ch1-Ch8 overrides to 110-810 respectively", but line 75 sets channel 4 to 4100 rather than 410. It is also outside the 1000-2000 RC PWM range, so it is not a plausible value in its own right.

The recorded sample output in docs/examples/channel_overrides.rst carried the same 4100 in three places. I regenerated those three lines from an actual Python 3 run rather than hand-editing them, which also fixes the key order: the old output was written under Python 2 dict ordering ('1', '3', '2', '5', '4', ...), and on Python 3.7+ dicts keep insertion order, so real output is now sequential.

Verified against dronekit-sitl copter (APM:Copter 3.3), exit 0:

Set Ch1-Ch8 overrides to 110-810 respectively
 Channel overrides: {'1': 110, '2': 210, '3': 310, '4': 410, '5': 510, '6': 610, '7': 710, '8': 810}

Branched off master so it is independent of #1253, though note that on current master import dronekit fails with ModuleNotFoundError: No module named 'past', so #1253 has to be applied before any example can actually be run.