fix typespecs for find_or_create

[CorneliaKelinske]

This fixes those two dialyzer complaints:

lib/actions.ex:191:invalid_contract
The @spec for the function does not match the success typing of the function.

Function:
EctoShorts.Actions.find_or_create/3

Success typing:
@spec find_or_create(Ecto.Query | Ecto.Schema, map(), Keyword.t()) :: {:error, _} | {:ok, %{atom() => _}}

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
lib/actions.ex:192:invalid_contract
The @spec for the function does not match the success typing of the function.

Function:
EctoShorts.Actions.find_or_create/2

Success typing:
@spec find_or_create(Ecto.Query | Ecto.Schema, map()) :: {:error, _} | {:ok, %{atom() => _}}