fix doc string for towel() map (#36)

The doc string for the `towel()` map has `(1+2z_n)` instead of `(1-2z_n)` as is implemented in the function body and the [referenced system](https://en.wikipedia.org/wiki/Hyperchaos#Mathematical_examples)

Author: ianwineman

src/discrete_famous_systems.jl

@@ -5,7 +5,7 @@ towel(u0 = [0.085, -0.121, 0.075])
 ```math
 \\begin{aligned}
 x_{n+1} &= 3.8 x_n (1-x_n) -0.05 (y_n +0.35) (1-2z_n) \\\\
-y_{n+1} &= 0.1 \\left[ \\left( y_n +0.35 \\right)\\left( 1+2z_n\\right) -1 \\right]
+y_{n+1} &= 0.1 \\left[ \\left( y_n +0.35 \\right)\\left( 1-2z_n\\right) -1 \\right]
 \\left( 1 -1.9 x_n \\right) \\\\
 z_{n+1} &= 3.78 z_n (1-z_n) + b y_n
 \\end{aligned}
@@ -579,4 +579,4 @@ end
     dx = mod(x + ν*(1 + μ*y) + ϵ*ν*μ*cos2pi(x), 1.0)
     dy = exp(-r)*(y + ϵ*cos2pi(x))
     return SVector{2}(dx, dy)
-end
+end