Update nucleotide-count description
Update nucleotide-count description

Points out that understanding DNA is not necessary to complete the
exercise, as well as making the input format more clear.

https://github.com/exercism/problem-specifications/issues/1716

Thanks to SleeplessByte, rpottsoh, kotp, ErikSchierboom, etc :)

https://github.com/exercism/problem-specifications/pull/1719

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