Prove that stopped discrete time nonnegative supermartingales are uniformly integrable

Question

I came across this note online. On page 40 theorem 2.2.18, it seems to suggest that any stopped discrete time nonnegative supermartingale is uniformly integrable and I cannot figure out why. To be more precise, let $\{X_{n}\}_{\mathbb{N}}$ be a discrete time nonnegative supermartingale and $T$ be a stopping time. Prove that the process $\{X_{T\land n}\}_{\mathbb{N}}$ is uniformly integrable. Thanks for your help!