proverb: Update description to describe inputs
proverb: Update description to describe inputs

- Eliminate bogus word 'horseshoe' from last line of example proverb
- Add text noting that the inputs may vary

https://github.com/exercism/problem-specifications/pull/994

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