tracing: Fix cpumask() example typo

tracing: Fix cpumask() example typo

[ Upstream commit eb9d58947d40699d93e5e69e1ddc54e41da7e132 ]

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>
Stable-dep-of: ea8d7647f9dd ("tracing: Verify event formats that have "%*p.."")
Signed-off-by: Sasha Levin <sashal@kernel.org>