docs: Fix wrong toHaveValue example (#431)

Small fix on README.md

The `.toHaveValue` example for the select verifies for `.not.toHaveValue`, but the code example has these two options selected.
That means it _should_ have this value, so I removed the `.not` part to have the test example match the code example.

Author: cbroeren

README.md

@@ -890,7 +890,7 @@ const selectInput = getByTestId('select-number')
 expect(textInput).toHaveValue('text')
 expect(numberInput).toHaveValue(5)
 expect(emptyInput).not.toHaveValue()
-expect(selectInput).not.toHaveValue(['second', 'third'])
+expect(selectInput).toHaveValue(['second', 'third'])
 ```
 
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