Fix doctest example since Inspect.MapSet changed (#13366)

Author: sabiwara

lib/ex_unit/lib/ex_unit/doc_test.ex

@@ -96,30 +96,30 @@ defmodule ExUnit.DocTest do
   values are treated as comments in Elixir code due to the leading
   `#` sign, they require special care when being used in doctests.
 
-  Imagine you have a map that contains a MapSet and is printed as:
+  Imagine you have a map that contains a `DateTime` and is printed as:
 
-      %{users: #MapSet<[:foo, :bar]>}
+      %{datetime: #DateTime<2023-06-26 09:30:00+09:00 JST Asia/Tokyo>}
 
   If you try to match on such an expression, `doctest` will fail to compile.
   There are two ways to resolve this.
 
   The first is to rely on the fact that doctest can compare internal
   structures as long as they are at the root. So one could write:
 
-      iex> map = %{users: Enum.into([:foo, :bar], MapSet.new())}
-      iex> map.users
-      #MapSet<[:foo, :bar]>
+      iex> map = %{datetime: DateTime.from_naive!(~N[2023-06-26T09:30:00], "Asia/Tokyo")}
+      iex> map.datetime
+      #DateTime<2023-06-26 09:30:00+09:00 JST Asia/Tokyo>
 
   Whenever a doctest starts with "#Name<", `doctest` will perform a string
   comparison. For example, the above test will perform the following match:
 
-      inspect(map.users) == "#MapSet<[:foo, :bar]>"
+      inspect(map.datetime) == "#DateTime<2023-06-26 09:30:00+09:00 JST Asia/Tokyo>"
 
   Alternatively, since doctest results are actually evaluated, you can have
-  the MapSet building expression as the doctest result:
+  the `DateTime` building expression as the doctest result:
 
-      iex> %{users: Enum.into([:foo, :bar], MapSet.new())}
-      %{users: Enum.into([:foo, :bar], MapSet.new())}
+      iex> %{datetime: DateTime.from_naive!(~N[2023-06-26T09:30:00], "Asia/Tokyo")}
+      %{datetime: DateTime.from_naive!(~N[2023-06-26T09:30:00], "Asia/Tokyo")}
 
   The downside of this approach is that the doctest result is not really
   what users would see in the terminal.