Update WIT.md to use func keyword instead of function

[eyebrowsoffire]

I have been familiarizing myself with the .wit format, and playing around with it in the online bindings generator. One thing I noticed is that the WIT.md grammar and examples all use the keyword function, but that doesn't actually work in the bindings generator, it uses the keyword func. I'm not sure if the documentation is out of date and the implementation is correct, or the other way around, but it would be good if these two things matched up.

[peterhuene]

It just changed to func very recently, so there's probably places where we failed to update the examples. If you spot any, we'd happily take a PR!

[eyebrowsoffire]

Oh it actually just updated. I was using a newer version of the online bindings generator but reading a version of WIT.md from yesterday! That being said, I did find that the grammar itself uses the wrong string, so I did open a PR: #238

[peterhuene]

Thanks again for putting up a fix for this issue!