tracing: Fix cpumask() example typo

The sample code for using cpumask used the wrong field for the
__get_cpumask() helper. It used "cpus" which is the bitmask (but would
still give a proper example) instead of the "cpum" that was there to be
used.

Although it produces the same output, fix it, because it's an example and
is confusing in how to properly use the cpumask() macro.

Link: https://lore.kernel.org/linux-trace-kernel/20221213221227.56560374@gandalf.local.home

Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>