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

kernel/trace/trace_events_filter.c

@@ -1816,7 +1816,7 @@ static void create_filter_finish(struct filter_parse_error *pe)
  * create_filter - create a filter for a trace_event_call
  * @tr: the trace array associated with these events
  * @call: trace_event_call to create a filter for
- * @filter_str: filter string
+ * @filter_string: filter string
  * @set_str: remember @filter_str and enable detailed error in filter
  * @filterp: out param for created filter (always updated on return)
  *           Must be a pointer that references a NULL pointer.