clearer formulations

src/lecture_7.md

@@ -26,7 +26,7 @@ Exercises 17.1–17.4, 18.1, 18.2
 
     After this lecture you will be able to:
 
-    - Describe what holes are, and apply this concept to describe the properties of semiconductors
+    - Describe the concept of holes, and apply this concept to describe the properties of semiconductors
     - Compute the density of states of electrons in semiconductors
     - Compute the density of charge carriers and chemical potential as a function of temperature