Chapter 18 missing reference to liquidity argument

[Nicolas-Mich3l]

In chapter 18 (generalized swapping), eventually swapMath.computeSwapStep is called with an argument: liquidity. Liquidity is not referenced / defined thus far. A look at the code indicates that liquidity should be an part of SwapState, yet SwapState, which is defined for the first time in this chapter does not have liquidity as a field.
As far as I understand it after looking at the code in the repo it is simply the current total liquidity in the pool. However the chapter does need revisions to clarify this for future readers so they don't need to go looking at the code.

The book has been a joy to follow along with btw :)