add other standard deviation

[shriram]

One of our stats teachers says:

I noticed the standard deviation function gives the population standard deviation and not the SAMPLE standard deviation which is used more in hypothesis testing

Since this is a topic BS:DS teachers want to cover, it would be good to have both.

[jpolitz]

Done:

import statistics as S

S.stdev-sample([list: 1, 2, 5, 6, 7])

(edited this comment after initially having the wrong function there)