fix(schematics): correct spec description in reducer template (#1269)

Author: koumatsumoto

modules/schematics/src/reducer/files/__name@dasherize@if-flat__/__name@dasherize__.reducer__dot__spec.ts

@@ -1,8 +1,8 @@
 import { reducer, initialState } from './<%= dasherize(name) %>.reducer';
 
 describe('<%= classify(name) %> Reducer', () => {
-  describe('unknown action', () => {
-    it('should return the initial state', () => {
+  describe('an unknown action', () => {
+    it('should return the previous state', () => {
       const action = {} as any;
 
       const result = reducer(initialState, action);