Fix link in source of zinc blende

src/extra_exercises.md

@@ -38,7 +38,7 @@ The figure below shows a conventional unit cell of one of the possible crystal s
 ![plan-view-zb](https://upload.wikimedia.org/wikipedia/commons/c/c0/Sphalerite-unit-cell-3D-balls.png)
 
 ??? info "source"
-    By Benjah-bmm27, own work, public domain, [link]([https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png)
+    By Benjah-bmm27, own work, public domain, [link](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png](https://commons.wikimedia.org/wiki/File:Sphalerite-unit-cell-3D-balls.png)
 
 1. What type of lattice does zincblende have? Give a set of primitive lattice vectors of this lattice.
 2. What is the basis of this crystal if we pick primitive lattice vectors? And what would be the basis if we pick conventional lattice vectors?