sched: Fix use of count for nr_running tracepoint

The count field is meant to tell if an update to nr_running
is an add or a subtract. Make it do so by adding the missing
minus sign.

Fixes: 9d246053 ("sched: Add a tracepoint to track rq->nr_running")
Signed-off-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200805203138.1411-1-pauld@redhat.com