Changed logistic model code to match description

- In the final exercise, in the exponent of the exponential, the
  parameter `a` is multiplying the other term
- However, in the accompanying code, the parameter `a` is dividing it
- The accompanying Wikipedia link about Item Response Theory also uses
  the convention of multiplying in the factor `a`
- Easy fix: in the code switch `a` from `a = 100` to `a = 1 / 100`, and
  switch it it multiplication in the assignment to `x`
- Closes AllenDowney#49