tracing: Update create_system_filter() kernel-doc comment

commit f306cc82 ("tracing: Update event filters for multibuffer")
added the parameter @tr for create_system_filter().

commit bb9ef1cb ("tracing: Change apply_subsystem_event_filter()
paths to check file->system == dir") changed the parameter from @system to @dir.

Link: https://lkml.kernel.org/r/20210325161911.123452-1-hqjagain@gmail.com



Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>