exercise 12: heading follows the new quote

SKILL.md

@@ -245,7 +245,7 @@ module the error localises to.
 
 For each spike or collapse, show the log rows before it. Say which column moved first.
 
-## 12. "The NN had learned something useless like time of day" (small)
+## 12. "The CNN has learned to detect a metal token" (small)
 
 > The CNN has learned to detect a metal token that radiology technicians place on the patient in the corner of the image field of view at the time they capture the image. -- Zech et al., whose pneumonia model scored AUC 0.931 in its own hospitals and 0.815 in someone else's