docs: rephrased text

lib/flame/component_controller.dart

@@ -25,7 +25,7 @@ abstract class ComponentController<T extends Component> extends Component {
 
   @override
   Future<void> add(Component component) {
-    throw Exception('ComponentController should not add other components.');
+    throw Exception('ComponentController cannot add other components.');
   }
 }
 

test/flame/component_controller_test.dart

@@ -47,7 +47,7 @@ void main() {
     );
 
     flameTester.test(
-      'throws Exception when adding other component',
+      'throws Exception when adding a component',
       (game) async {
         final component = ControlledComponent();
         final controller = TestComponentController(component);