fix: Remove argumentError doctest

Attempted to run the original code, and tried this as well but the argument error is still propagated to the livebook, and kills compilation.

```elixir
  iex> assert_raise ArgumentError, fn -> %Character{} end
  :ok
```

exercises/rpg_dialogue.livemd

@@ -90,13 +90,10 @@ defmodule Character do
 
   Defines a character struct, and functions for creating character dialogue.
 
-  iex> %Character{name: "Frodo"}
-  %Character{name: "Frodo", weapon: nil, class: nil}
+  iex> %Character{name: "Frodo", weapon: "Sting"}
+  %Character{name: "Frodo", weapon: "String", class: nil}
 
-  The :name key is required.
-
-  iex> %Character{name: "Frodo"}
-  ** (ArgumentError) the following keys must also be given when building struct Character: [:name]
+  The :name key is required. But cannot be tested by doctests
   """
   defstruct []