Fix example plot of Laplace distribution

In the PDF used for the example plot of the Laplace distribution only the location parameter loc instead of the absolute difference between random variable and loc was divided by the scale parameter. For the example at hand this makes no difference as loc is 0 and lambda is 1. For different values, however, the plot makes no sense.
Merge pull request #3919 from nkoep/master

Fix example plot of Laplace distribution