remove link to exercises

README.md

@@ -4,7 +4,7 @@
 
 * 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).
+  Exercises: 2.3, 2.4, 2.5, 2.6, 2.8
 
 * Week 2: [Free electron model](lecture_2.md)  
   (based on chapters 3–4)