Chapter 2: n-body problem code needs adjusting

[doubledherin]

Hey there! We started referring to "movers" as "bodies" here, but forgot to update the last 3 references to "movers":

In addition, there is an earlier code sample (see below) in which the applyForce method is invoked within the Body attract method. Since we're also calling applyForce in the code above, we should update the below code snippet to simply return force instead of applying it.