This is more brilliant than I realized at first. There's a hidden twist.
When Raven says "I stand corrected," at first I thought that meant she was corrected from thinking Gwen can't dance to thinking Gwen can dance. But Raven never said Gwen can't dance. What Raven said was "sure," implying that Raven was agreeing with Gwen that Gwen is a good dancer. So then when Raven says "I stand corrected," she's secretly calling Gwen a bad dancer without Gwen even knowing it. Even though Raven's "sure" was half-hearted and Gwen interpreted it as disbelief, Raven was still able to say she stands corrected about having agreed with Gwen that she can dance, meaning she now thinks Gwen can't dance. The hidden insult is betrayed only by Raven's smile in that final frame.