Book 2: improve center_vec name

[whydoubt]

As we know, naming variables is one of the hardest things...

In listing 2.3, center_vec is used to indicate the change
in position over an interval of time. I don't feel the name
conveys that well. Also, there is a well-established name
for that concept: velocity. So I would propose velocity
as a replacement. Note that this would also need to be
changed in listings 2.10 and 2.11.

Also in listing 2.11... Some underscores appeared where
they weren't before: center_vec = _center2 - _center1;
Looks like it may have been a leftover from an earlier
revision of the book.

[whydoubt]

A step further could be to pass velocity instead of center2... cutting out the intermediary.
I haven't yet checked if that would complicate anything further down in the text though.
I could put together a PR to this effect if the idea seems good.

[whydoubt]

If you only want to consider the change in position, without the hard implication that velocity is constant, displacement could be another alternative for center_vec.