fig_2_14 parallel_scan example code fails for non_zero v[0]

[jnorwood]

Using the book example code, I modified the initial values of input vector v to be i+2 instead of i. Running the serial sum returns 44 as last value, while the parallel_scan sum returns 42.

Fix for the fig_2_14 function which contains the parallel_scan is to remove the rsum[0] = v[0] an change the blocked_range to (0, N).

With those changes I get matching result for serial_rsum and parallel_rsum vectors

`
For N=8
input 2,3,4,5,6,7,8,9

combine (x=0,y=0)
sum[0..1), fs=True, 2
sum[2..3), fs=False, 4
sum[3..4), fs=False, 5
sum[6..7), fs=False, 8
combine(x=4,y=5)
sum[5..6), fs=False, 7
sum[1..2), fs=False, 3
sum[4..5), fs=False, 6
combine(x=2,y=3)
combine(x=6,y=7)
combine(x=5,y=9)
combine(x=14,y=13)
sum[1..2), fs=True, 5
combine(x=5,y=4)
sum[3..4), fs=True, 14
combine(x=14,y=6)
sum[2..3), fs=True, 9
combine(x=27,y=8)
sum[4..5), fs=True, 20
sum[7..8), fs=True, 44
sum[5..6), fs=True, 27
sum[6..7), fs=True, 35

parallel_sum = 2,5,9,14,20,27,35,44
`