Update index.mdx

Fix typo

public/content/lessons/2017/area-and-slope/index.mdx

@@ -30,7 +30,7 @@ Before jumping into the main line of reasoning, the way this works is to take th
 
 <Figure
     image="figures/quick-intuition.svg" 
-    caption="Imagine the area as a pile of that when you shake it all down to be flat, forms a rectangle between the two points."
+    caption="Imagine the area as a pile of sand that when you shake it all down to be flat, forms a rectangle between the two points."
 />
 
 This is intuitive, but you might find it worthwhile to also think about how this relates to the more familiar method of averaging finitely many numbers, which is to add them all up and divide by how many there are. A very common feeling is that when you use a sum in a discrete context, you use an integral in a continuous context. But to avoid subtle errors in making this translation, its useful to be able to think exactly how and why the analogy from sums to integrals works in various contexts where it comes up. This question of finding the average value of a function offers a great chance exercise that muscle.