[FIX] Fixed type: struct action to enum action (pointfreeco#2144)

Author: Jaesung

Sources/ComposableArchitecture/Documentation.docc/Articles/MigratingToTheReducerProtocol.md

@@ -274,7 +274,7 @@ like so:
 struct FeatureState { 
   // ...
 }
-struct FeatureAction { 
+enum FeatureAction { 
   // ...
 }
 struct FeatureEnvironment {