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>
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