shorten the TOC

SUMMARY.md

 # Summary
 
-* Week 1: [Phonons and specific Heat](lecture_1.md)  
-  (based on chapter 2 of the book)  
-  Exercises: 2.3, 2.4, 2.5, 2.6, 2.8 (get them [here](homeworks/homework_week_1.pdf) if you don't have the book yet).
-* Week 2: [Free electron model](lecture_2.md)  
-  (based on chapters 3–4)  
-  Exercises 3.1, 3.3, 4.2, 4.5, 4.6, 4.7
-* Week 3: [Atoms and bonds](lecture_3.md)  
-  (based on chapters 5–8)  
-  Exercises 5.2, 6.2, 6.3, 6.4, 6.5
-* Week 4: [Electrons and phonons in 1D](lecture_4.md)  
-  (based on chapters 9–11) <br>
-  Exercises 9.2, 9.4, 10.1, 10.2, 11.2, 11.5
-* Week 5: [Crystal structure and diffraction](lecture_5.md)  
-  (based on chapters 12–14, up to and including 14.2)  
-  Exercises 12.3, 12.4, 13.3, 13.4 & 14.2
-<!-- * Week 6: Tight binding and nearly free electrons   -->
-<!--   (based on chapters 15–16) -->
-<!-- * Week 7: Semiconductors   -->
-<!--   (based on chapters 17–18) -->
-<!-- * Week 8: Magnetism   -->
-<!--   (based on chapters 19-20) -->
+* [Phonons and specific Heat](lecture_1.md)
+* [Free electron model](lecture_2.md)
+* [Atoms and bonds](lecture_3.md)
+* [Electrons and phonons in 1D](lecture_4.md)
+* [Crystal structure and diffraction](lecture_5.md)
+* Tight binding and nearly free electrons
+* Semiconductors
+* Magnetism