Exercise asks developer to write a function

Having an exercise direct a student to write a function or write a
method or write "some specific structure" can be confusing when the
language they are solving the exercise in does not have that structure.

Where we say things like "Create a function" or "Write a function" we
could be indicating that we need a solution instead. This will
encourage the student to determine what that solution is,
rather than us telling them to specifically create a function.

Unless the exercise is a concept exercise about functions, of course,
then the student should be told to create a function.

The exercises that mention "create a function" are:

- [ ] exercises/parallel-letter-frequency/description.md

And the exercises that mention "Write a function" are:

- [ ] exercises/sublist/metadata.toml
- [ ] exercises/flatten-array/description.md
- [ ] exercises/nucleotide-codons/metadata.toml
- [ ] exercises/nucleotide-codons/description.md
- [ ] exercises/alphametics/metadata.toml
- [ ] exercises/alphametics/description.md
- [ ] exercises/go-counting/description.md
- [ ] exercises/darts/description.md
- [ ] exercises/darts/metadata.toml

ref: [exercism/ruby#1638-comment](https://github.com/exercism/ruby/pull/1638#discussion_r1501824390)