tracing: Fix spelling mistake "aritmethic" -> "arithmetic"

There is a spelling mistake in the tracing mini-HOWTO text. Fix it.

Link: https://lkml.kernel.org/r/20211108201513.42876-1-colin.i.king@gmail.com

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>