This PR contains minor updates to the enums article I originally contributed in #45.

I made changes to the writing (e.g. "exhaustiveness checking" instead of "exhaustiveness") and
I made the following changes:

• use the plural of enum constants in a comment about listing them
• use the name "exhaustiveness checking" (should that be Title Case?) instead of just the concept of exhaustiveness as well as add a link to the patternmatching article (The article isn't available in this repository so I didn't use an id: link)
• move the enum constants of the enum with more than the enum declaration in the class body to two lines. This makes it easier to see the ; after the enum declaration.
• add a few spaces in code as most people prefer that I think
• slightly reword the conclusion