docs: fix the example for test.fails function

[vasilii-kovalev]

Link to the docs.

1. The original example produces the following output to the console:
   
2. After changing the example according to the rejects assertion's description (adding await before expect and making the whole test asynchronous), the output to the console changes to this:
   
3. When changing test.fails to test, the output is the following:
   

If I understand correctly, in the example the assertion throws an error, that a promise is resolved instead of being rejected, but since we use .fails function, vitest will mark the test as passed. If so, it seems the output of the fixed example (2) is correct.

If I understand the idea of test.fails incorrectly, I think it will be useful to leave a note in the dedicated discussion and/or in this pull request's comments.

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