After a red-hot run, are investors giving up on flying taxi stocks?Joby Aviation (JOBY -11.12%) stock is falling in Monday's trading. The flying electric vehicle company's share price was down 10.2% as of 1 p.m. ET.After an impressive rally, Joby Aviation's valuation is now falling as some investors are betting against the stock. Short-sellers are also placing bets against Joby's competitor Archer Aviation, and its stock is down 23.7% today as of this writing. Even with today's pullback, Joby's share price ...