docs: Add test scenario to exercise in queueing-a-series-of-state-updates.md

[santiph]

I've found a wrong solution through a reduce() operation when solving this exercise as follows:

export function getFinalState(baseState, queue) {
  let finalState = queue.reduce((accumulator, currentChange) => {
    if ('function' === typeof currentChange) {
      return currentChange(accumulator);
    } else {
      return baseState + currentChange;
    }
  }, baseState);

  return finalState;
}

All tests were passing successfully. The return statement for queued numbers should've been this instead: return currentChange;

This change introduces a new test case to fail in this case :')

[github-actions[bot]]

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