Zemmels review: use word expression instead of formula

From Review #96 (closed):

I'm splitting hairs here, but I believe the use of the word "formula" in the Streamflow Percentiles is incorrect. A formula expresses the relationship between quantities like "y = mx + b." I think a more appropriate term here is "expression," unless you were to set the provided expression equal to a symbol representing the plotting position like "p = \frac{i}{n + 1}". Changed to 'expression', thanks for your mathematical expertise!