Use defined variable

The last example showing that `:oops` is not a valid key uses the undefined variable `user` instead of the defined `User`s, either `jose` or `eric`, so instead of `ArgumentError` being raised, a `RuntimeError` is raised:

```iex
iex(26)> %{user | oops: :field}
** (RuntimeError) undefined function: user/0
iex(26)> %{eric | oops: :field}
** (ArgumentError) argument error
    (stdlib) :maps.update(:oops, :field, %User{age: 27, name: "eric"})
```