Update nested_loop_number_of_failed_students_example.py

Created a more meaningful variable name

Author: dphoenix

python-break/nested_loop_number_of_failed_students_example.py

@@ -3,8 +3,8 @@
 
 failed_score = 60
 num_failed_students = 0
-for score_set in scores:
-    for score in score_set:
+for student_score_list in scores:
+    for score in student_score_list:
         if score < failed_score:
             num_failed_students += 1
             break