remove extra space which raise warning

source/Collection_Data_Types.rst

@@ -105,7 +105,7 @@ wihout using python shell, what is the results of the following statements:
    sum is a function which return the sum of each elements of a list.
       
 ::
- 
+
    x = [1, 2, 3, 4]
    x[3] = -4 # what is the value of x now ?
    y = sum(x)/len(x) #what is the value of y ? why ?