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>

Documentation/trace/ftrace.rst

@@ -2574,7 +2574,7 @@ want, depending on your needs.
 
 - The cpu number on which the function executed is default
   enabled.  It is sometimes better to only trace one cpu (see
-  tracing_cpu_mask file) or you might sometimes see unordered
+  tracing_cpumask file) or you might sometimes see unordered
   function calls while cpu tracing switch.
 
 	- hide: echo nofuncgraph-cpu > trace_options