Update lecture_6.md - minor change in LO

src/lecture_6.md

@@ -15,11 +15,9 @@ Exercises 15.1, 15.3, 15.4, 16.1, 16.2
     After this lecture you will be able to:
 
     - formulate a general way of computing the electron band structure - the **Bloch theorem**.
-    - discuss that in a periodic potential all electron states are Bloch waves.
+    - recall that in a periodic potential all electron states are Bloch waves.
     - derive the electron band structure when the interaction with the lattice is weak using the **Nearly free electron model**.
 
-    
-
 Let's summarize what we learned about electrons so far:
 
 * Free electrons form a Fermi sea ([lecture 2](lecture_1.md))