Wrong variable name in docs for Access.get_and_update/3

At a glance I can't figure out why this is so, since I'm not familiar with how these get generated, but the short signature for [`c:Access.get_and_update/3`](https://hexdocs.pm/elixir/Access.html#c:get_and_update/3) is called `list` [when it should be](https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/access.ex#L202) `fun`.