fix: update the failing config example in Readme

[rajrajhans]

• If the config is taken as is from the README, it fails with the following error:

      ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:system, "AWS_ROLE_ARN"} of type Tuple. This protocol is implemented for the following type(s): Atom, BitString, CloudfrontSigner.Policy, Date, DateTime, Decimal, Float, Hex.Solver.Assignment, Hex.Solver.Constraints.Empty, Hex.Solver.Constraints.Range, Hex.Solver.Constraints.Union, Hex.Solver.Incompatibility, Hex.Solver.PackageRange, Hex.Solver.Term, Integer, List, NaiveDateTime, Phoenix.LiveComponent.CID, Postgrex.Copy, Postgrex.Query, Time, URI, Version, Version.Requirement
          (elixir 1.14.1) lib/string/chars.ex:3: String.Chars.impl_for!/1
          (elixir 1.14.1) lib/string/chars.ex:22: String.Chars.to_string/1
          (elixir 1.14.1) lib/uri.ex:153: URI.encode_kv_pair/2
          (elixir 1.14.1) lib/enum.ex:1755: anonymous fn/2 in Enum.map_join/3
          (elixir 1.14.1) lib/enum.ex:1725: anonymous fn/4 in Enum.map_intersperse/3
          (stdlib 4.1) maps.erl:411: :maps.fold_1/3
          (elixir 1.14.1) lib/enum.ex:2480: Enum.map_intersperse/3
          (elixir 1.14.1) lib/enum.ex:1755: Enum.map_join/3

This PR updates the example so that it will work correctly without error.