tests: change mockImplementationOnce to mockResolvedValueOnce (#83)

Author: Gpx

README.md

@@ -130,11 +130,7 @@ import Fetch from '../fetch' // see the tests for a full implementation
 
 test('Fetch makes an API call and displays the greeting when load-greeting is clicked', async () => {
   // Arrange
-  axiosMock.get.mockImplementationOnce(() =>
-    Promise.resolve({
-      data: {greeting: 'hello there'},
-    }),
-  )
+  axiosMock.get.mockResolvedValueOnce({ data: { greeting: 'hello there' } })
   const url = '/greeting'
   const {getByText, getByTestId, container} = render(<Fetch url={url} />)