message: Update return_tip to drop_tip
diff: @@ -46,7 +46,7 @@ This accomplishes the same thing as the following basic commands:
  p300.pick_up_tip()
  p300.aspirate(100, plate.wells('A1'))
  p300.dispense(100, plate.wells('B1'))
  - p300.return_tip()
  + p300.drop_tip()
  ******************************
  