fix lint error

Author: jzlxiaohei

src/createStore.js

@@ -62,7 +62,7 @@ export default function createStore(reducer, initialState) {
 
     return function unsubscribe() {
       var index = listeners.indexOf(listener);
-      if(index === -1)return;
+      if (index === -1)return;
       listeners.splice(index, 1);
     };
   }

test/createStore.spec.js

@@ -206,7 +206,7 @@ describe('createStore', () => {
     const listenerA = expect.createSpy(() => {});
     const listenerB = expect.createSpy(() => {});
 
-    let unsubscribeA = store.subscribe(listenerA);
+    const unsubscribeA = store.subscribe(listenerA);
     store.subscribe(listenerB);
 
     unsubscribeA();