corrected exercice finding missing element in list

ipynb/pres02_basic_statements.ipynb

@@ -1140,10 +1140,12 @@
 ``` python
 from random import shuffle, randint
 
 n = 20
 i = randint(0, n-1)
+# i = 19 #special case for testing, element at the end of the list
+
 print('integer remove from the list:', i)
 l = list(range(n))
 l.remove(i)
 shuffle(l)
 print('shuffled list: ', l)
@@ -1172,11 +1174,11 @@
     if elem != idx:
         found = idx
         break
 if found is None:
     found = len(l)
-print("missing ", idx)
+print("missing ", found)
     
 ```
 
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