Different variable names from type variables

[fdbeirao]

I am writing a small ELM beginners guide. I am at the part of explaining pattern matching. While trying to explain this example, I feel that some people might confuse a and b from the type signature (Either a b) with this a and b from the pattern matches. I tried to come up with a more descriptive name, just to avoid an unnecessary clash.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.