workqueue: Fix cpu_intensive_thresh_us name in help text

There exists no parameter called "cpu_intensive_threshold_us".
The actual parameter name is "cpu_intensive_thresh_us".

Fixes: 63638450 ("workqueue: Report work funcs that trigger automatic CPU_INTENSIVE mechanism")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>