scale-generator: fix name of augmented interval
scale-generator: fix name of augmented interval

The interval described is an augmented *second*, not an augmented first.
Added also its composition in terms of steps for completeness.

https://github.com/exercism/problem-specifications/pull/1643
https://github.com/exercism/problem-specifications/issues/1642

Signed-off-by: Rafael Fonseca <92b5eb2fd9104ba50cbd70c67dc2a52019d1ffde@gmail.com>

petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust