Fix Jest example's usage of default imports

Default exports and imports are deprecated. The default import of `zustand` generates a deprecation warning on the console. The default export that was suggested previously would actually fail to compile. See the comments at the very bottom of #559 for further discussion.

This commit fixes the example in the documentation by introducing named exports & imports instead of default exports & imports.

docs/guides/testing.mdx

@@ -12,7 +12,7 @@ Thus, there can be cases where the state of one test can affect another. To make
 
 ```jsx
 import { create as actualCreate } from 'zustand'
-// const actualCreate = jest.requireActual('zustand') // if using jest
+// const { create: actualCreate } = jest.requireActual('zustand') // if using jest
 import { act } from 'react-dom/test-utils'
 
 // a variable to hold reset functions for all stores declared in the app
@@ -42,7 +42,7 @@ If you are using zustand, as documented in [TypeScript Guide](./typescript.md),
 
 ```tsx
 import { create as actualCreate, StateCreator } from 'zustand'
-// const actualCreate = jest.requireActual('zustand') // if using jest
+// const { create: actualCreate } = jest.requireActual('zustand') // if using jest
 import { act } from 'react-dom/test-utils'
 
 // a variable to hold reset functions for all stores declared in the app
@@ -70,13 +70,13 @@ You should use the following code in the `__mocks__/zustand.js` file (the `__moc
 
 ```js
 import { act } from '@testing-library/react-native'
-const actualCreate = jest.requireActual('zustand')
+const { create: actualCreate } = jest.requireActual('zustand')
 
 // a variable to hold reset functions for all stores declared in the app
 const storeResetFns = new Set()
 
 // when creating a store, we get its initial state, create a reset function and add it in the set
-const create = (createState) => {
+export const create = (createState) => {
   const store = actualCreate.default(createState)
   const initialState = store.getState()
   storeResetFns.add(() => {
@@ -93,8 +93,6 @@ beforeEach(async () => {
     })
   )
 })
-
-export default create
 ```
 
 If the `jest.config.js` has `automock: false`, then you need to do the following in `jest.setup.js`: