Shares of Zipcar Inc. plunged after CNBC Mad Money host Jim Cramer said growth had already peaked for the Cambridge car-sharing firm. Zipcar, however, recovered later in the session to end higher. “I just don’t see how tiny Zipcar with its $400 million market cap can compete against these titans,” Cramer said, referring to firms like Hertz Global Holdings. Cost-conscious customers and rising gas prices have driven growth in the car-sharing market, attracting new entrants such as Hertz.