fix documented equation for diskio average queue depth

[phemmer]

I got the documented equation wrong for calculating the average queue depth. I based it off the equation in iostat, but the iostat /1000 was for the time unit conversion. For us this is handled by the derivative(..., 1ms).

Demonstration of the correct equation:

# iostat -t -d -x 15 /dev/sdb
Linux 3.10.0-514.21.2.el7.x86_64 (fll2gdbs10stg) 	10/12/17 	_x86_64_	(2 CPU)

...

10/12/17 17:08:32
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0.00     4.47 1479.13   12.33 756996.80   117.33  1015.26     9.39    6.30    6.34    1.26   0.65  96.96

10/12/17 17:08:47
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0.00     3.13 1602.20   13.07 820326.40   129.87  1015.88    10.37    6.42    6.47    0.91   0.60  97.19

10/12/17 17:09:02
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0.00     2.60 1741.87   10.80 891801.87   105.07  1017.77    10.57    6.03    6.06    0.83   0.55  96.77