Documentation: Fix filename typo in ftrace doc

The filename for setting the cpumask should be `tracing_cpumask`,
instead of `tracing_cpu_mask`.

Signed-off-by: Yuanhsi Chung <freshliver.cys@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20231104103329.215139-1-freshliver.cys@gmail.com>