tracing: Fix comments of create_filter()

The name in comments of parameter "filter_string" in function
create_filter is annotated as "filter_str", just fix it.

Link: https://lkml.kernel.org/r/20220524063937.52873-1-sunliming@kylinos.cn

Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>