CHANGE: Change parameter of Zeta distribution from `a` to `s`

[MichaelOwenDyer]

The distribution rand_distr::zipf::Zeta uses a as its parameter name, but s is the standard name (on Wikipedia and in the implementation of its sister distribution zipf::Zipf).

Summary

Correcting this would be a breaking change since ZetaError::ATooSmall is pub and potentially depended upon.

Details

• Refactor Zeta attribute a_minus_one to s_minus_one
• Refactor ZetaError::ATooSmall to ZetaError::STooSmall (and update Display impl)
• Update documentation / comments

Motivation

There is possibly not enough reason to justify this breaking change, as the implementation is not broken by any means. It would be merely a matter of consistency.

Alternatives

• Just leave it the way it is

[dhardy]

It's a very small breaking change. I think we should go ahead.