In the final exercise for Chapter 10, there is a slight mismatch between the definition of the 3-parameter logistic function and its implementation. In the definition, we see that the parameter a is multiplying the other term in the exponent of the exponential.

However, in the implementation in the next cell, we see that the variable a is dividing the rest of the terms (in the assignment to x).

The Wikipedia link also uses the convention of having the parameter a multiply the terms. I believe that the easiest solution would be to change the value of a to 1/100 and change it to multiplication in the definition of x.