fix doc (#3631)

Author: arnauddorgans

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

@@ -199,8 +199,7 @@ Instead, we recommend invoking the child reducer directly:
 
 ```swift
 case .buttonTapped:
-  return Child().reduce(into: &state.child, action: .refresh)
-    .map(Action.child)
+  return reduce(into: &state, action: .child(.refresh))
 ```
 
 ### CPU intensive calculations