Docs: remove reference to Key

[fsackur]

Hiya,

I understand that Key has been removed from the library.

This took me a bit of headscratching to figure out! I was following https://github.com/alecthomas/injector/blob/master/docs/terminology.rst

Github won't let me easily link the line number, but it's under "Assisted Builder" and the line looks like this: >>> DB = Key('DB')

Could we get the docs updated with the new way of doing things?

I'm new to this library, so if you'd like me to PR it, then I'll need guidance on the correct content.

[jstasiak]

Hey,

Hmm, yeah – I somehow missed this when changing stuff around. I'll update this piece of documentation when I come around to work in the project again (PRs are welcome and I'd give some advice about the correct content there but I don't remember this section too well right now).