Fix tf.meshgrid documentation (#9740)

The original document has the wrong output regarding the default indexing xy.

    x = [1, 2, 3]
    y = [4, 5, 6]
    X, Y = tf.meshgrid(x, y)
    print(X.eval())
    print(Y.eval())

    [[1 2 3]
     [1 2 3]
     [1 2 3]]
    [[4 4 4]
     [5 5 5]
     [6 6 6]]