Change example to one that works

Struct.new used to raise an error in Ruby 3.2 and earlier, but in 3.3 it's allowed.
So we make a new class that actually requires an arg

spec/unit/dry/cli/registry_spec.rb

@@ -28,7 +28,9 @@
 
     context "when class is given" do
       it "raises error when #initialize arity is not equal to 0" do
-        callback = Struct
+        callback = Class.new do
+          def initialize(foo); end
+        end
 
         expect do
           Bar::CLI::Commands.before("alpha", callback)
@@ -64,7 +66,9 @@
 
     context "when class is given" do
       it "raises error when #initialize arity is not equal to 0" do
-        callback = Struct
+        callback = Class.new do
+          def initialize(foo); end
+        end
 
         expect do
           Bar::CLI::Commands.after("alpha", callback)