fix cpu load calculation

The `cpuwidget` considers *system load* (i.e. time spent in kernel mode) as idle time, probably due to a simple array subscript typo (indexing from 0 instead of 1). This causes the widget to show less CPU utilisation than it is supposed to.

For **example**, a workload that causes 75% user-space and 25% kernel-space load will be reported as 75% CPU utilisation by the widget.

See `proc(5)` for more.