Add a scalar product to the example list comprehensions based on a suggestion

sent to python-docs.

Doc/tut/tut.tex

@@ -1846,6 +1846,8 @@ SyntaxError: invalid syntax
 [8, 6, -18, 16, 12, -36, 24, 18, -54]
 >>> [x+y for x in vec1 for y in vec2]
 [6, 5, -7, 8, 7, -5, 10, 9, -3]
+>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
+[8, 12, -54]
 \end{verbatim}