Fix link to Wumbo

public/content/lessons/2017/derivatives-trig-functions/index.mdx

@@ -14,7 +14,7 @@ credits:
 
 Let's try to reason through what the derivatives of the functions sine and cosine should be. For background, you should be comfortable with how to think about both of these functions using the unit circle; that is, the circle with radius $1$ centered at the origin[^1].
 
-[^1]: To get up to speed on the unit circle or for a quick referesher see [this article](https://wumbo.net/concept/unit-circle/).
+[^1]: To get up to speed on the unit circle or for a quick referesher see [this article](https://wumbo.net/concepts/unit-circle/).
 
 For example, how would you interpret the value $\sin(0.8)$ if the value $\theta = 0.8$ is understood to be in radians?
 You might imagine walking around a circle with a radius of $1$, starting from the rightmost point, until you’ve traversed the distance $0.8$ in arc length.