[Elixir] Fine-tune wording across all exercises

[angelikatyborska]

This PR is mostly an indulgence of my obsessive need to have everything consistent and procrastinating on writing concept exercises 👻. It does two things:

• Replaces the usage of the word "parameter" with "argument". In many other programming languages those are considered to be synonyms, but Elixir is pretty consistent about exclusively using the word "argument" to describe the data passed into functions. Normally I wouldn't care who uses which word, but we're trying to teach here and I think it might help some beginner programmers if we use the same wording as the docs and the error messages (e.g. ArgumentError) do.
• Updates the error message in boilerplate files to always follow the pattern "Please implement the foo/n function".