small style fix

src/13_semiconductors.md

@@ -50,7 +50,7 @@ An empty band is not gonna lead to any interesting physical properties no matter
 Therefore, it is also important *how* bands are filled by the particles.
 By carefully controlling the distribution of particles in the bands, we are able to engineer material properties that we require.
 Without a doubt, the greatest example is *semiconductors* - the bedrock of all electronics. 
-In this lecture, we shall grasp the basics of semiconductors by learning how to treat bands at different filling.
+In this lecture, we shall grasp the basics of semiconductors by learning how to treat bands at different levels of filling.
 
 ## Review of band structure properties