docs: ✏️ fix "handle submit" test and add expectation (#186)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**:

The "clicks submit button" test was not working because it was missing
`handleClick`. Also, it did not have any expectation which is weird for
a test.

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**Why**:

I think having the test more "real-worldly" will make it easier to understand.

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- [ ] Tests N/A
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Author: Gpx

README.md

@@ -607,9 +607,11 @@ import {render, cleanup, fireEvent} from 'react-testing-library'
 afterEach(cleanup)
 
 test('clicks submit button', () => {
+  const handleClick = jest.fn()
   const {getByText} = render(<button onClick={handleClick}>Submit</button>)
 
   fireEvent.click(getByText('Submit'))
+  expect(handleClick).toHaveBeenCalledTimes(1)
 })
 ```