Fix incorrect formula for mean in geometric.jl #1

[mmogib]

This pull request corrects the formula for the mean of the geometric distribution in geometric.jl.

The current formula is incorrect. The correct formula for the mean of a geometric distribution is:

$$ \mu = \frac{1}{p} $$

where $p$ is the probability of success.

References:

1. Montgomery, D. C., & Runger, G. C. (2011). Applied Statistics and Probability for Engineers (5th ed.). Wiley. (p. 88)

2. Wikipedia contributors. (2025, May 20). Geometric distribution. In Wikipedia, The Free Encyclopedia. Retrieved June 8, 2025, from https://en.wikipedia.org/w/index.php?title=Geometric_distribution&oldid=1291255291

[devmotion]

The formula is actually correct: The Geometric distribution in Distributions.jl models the number of failures until the first success with support 0, 1, 2, .... (right column in the table on the wikipedia page and the second definition discussed in the text). This distribution has mean (1-p)/p where p denotes the success probability, which is exactly how mean is currently defined in Distributions.jl.