Use `.city` instead of `[:city]` in history test

In the `get history of an item` test, I got the following:
```1) test get history of an item (Append.AddressTest)
test/append/address_test.exs:56
** (UndefinedFunctionError) function Append.Address.fetch/2 is undefined (Append.Address does not implement the Access behaviour)
code: assert h1[:city] == "Asgard"
stacktrace:
(append) Append.Address.fetch(%Append.Address{__meta__: #Ecto.Schema.Metadata<:loaded, "addresses">, address_line_1: "The Hall", address_line_2: "Valhalla", city: "Asgard", entry_id: "b0cccc58-f3ea-4b9d-9b57-9b79c2bbf69a", id: 100, inserted_at: ~N[2018-12-29 20:29:29.782939], name: "Thor", postcode: "AS1 3DG", tel: "0800123123", updated_at: ~N[2018-12-29 20:29:29.782939]}, :city)
(elixir) lib/access.ex:318: Access.get/3
test/append/address_test.exs:70: (test)
```
In my own code I fixed this by changing the assert statements to check `h1.city` instead of `h1[:city]` (and likewise for h2).