Changed logistic model code to match description

[EricRobertCampbell]

• 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 to multiplication in the assignment to x
• Code change:
• Closes AllenDowney/ThinkBayes2#49