Updated return_tip to drop_tip

Comments said that the two code snippet accomplished the same thing. The first (p300.transfer(100, plate['A1'], plate['B1'])) would drop the tip in the trash while the second   

p300.pick_up_tip()
p300.aspirate(100, plate.wells('A1'))
p300.dispense(100, plate.wells('B1'))
p300.return_tip()

would return the tip to the tiprack. I changed return_tip in the second example to drop_tip so the two commands would be identical.

api/docs/v2/new_examples.rst

@@ -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()
 
 ******************************