Update Mixer.md

[jbussey-lv]

Corrected range that corresponds to percents to be listed as -100 to 100

[sensei-hacker]

Thanks for looking at this! An smix value outside the range -100 to +100 is unusual, but allowed. The range is indeed -1000 to +1000.
See line #2012 of src/main/fc/cli.c

Analogously, the default tricopter mix sets the rear motor to 133.3% and the two front motors 66.7%, because the two front motors carry the same weight as the single rear motor.

The document could be clarified, perhaps, to make it very clear that 100 is 100% - yet you can set 200% if you choose to. Of course that will mean the movement saturates if the input goes past 50%.