Tutorial 4 Should Reference ifCond.

[joshpoll]

Tutorial 4 currently says that if statements are not allowed in constraint definitions. While it is true that JS if statements are not allowed, it seems that Autodiff.ts does have an ifCond function that can be used to construct a constraint with a conditional: https://github.com/penrose/penrose/blob/dbf710646bcaad9f0e3142388ae4759ca6b3a740/packages/core/src/engine/Autodiff.ts#L798