Fix "behavior" spelling

[keijokapp]

Closes #4

[garyb]

I (British) made this kind of mistake so many times!

[toastal]

@garyb If it's any consolation, Erlang seamlessly supports both behavior and behaviour as keywords without acting like one of the spellings is more correct.

[keijokapp]

As I understand, the goal here is to mimic web standards as closely as possible and the standard says behavior. I don't speak neither English natively and don't have any preference aside from what the standard says. I don't think the effort and complexity of supporting both variants (and deviating from the standard by doing so) is warranted.

[toastal]

Agreed it's not worth it in the `Web` libraries as they're meant to follow specs. It's just an unfortunate part of programming that what is the same word but different spelling often isn't supported.