Fixed erroneous output and added a little clarity on the matter

list.index(argument) would return the index of the item in the list that first matched the argument 
It will not return the value stored at the index of the argument as it was prior.
Added some more clarity to the subject as well.

python3.html.markdown

@@ -224,8 +224,8 @@ li.remove(2)  # Raises a ValueError as 2 is not in the list
 # Insert an element at a specific index
 li.insert(1, 2)  # li is now [1, 2, 3] again
 
-# Get the index of the first item found
-li.index(2)  # => 3
+# Get the index of the first item found matching the argument
+li.index(2)  # => 1
 li.index(4)  # Raises a ValueError as 4 is not in the list
 
 # You can add lists