tracing/fprobe-event: cleanup: Fix a wrong comment in fprobe event

Despite the fprobe event,  "Kretprobe" was commented. So fix it.

Link: https://lore.kernel.org/all/170952361630.229804.10832200172327797860.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>