bpftool: fix perf help message

Currently, bpftool perf subcommand has typo with the help message.

$ tools/bpf/bpftool/bpftool perf help
Usage: bpftool perf { show | list }
bpftool perf help }

Since this bpftool perf subcommand help message has the extra bracket,
this commit fix the typo by removing the extra bracket.

Signed-off-by: Daniel T. Lee <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/bpf/bpftool/perf.c

···
 {
 	fprintf(stderr,
 		"Usage: %1$s %2$s { show | list }\n"
-		"       %1$s %2$s help }\n"
+		"       %1$s %2$s help\n"
 		"\n"
 		"       " HELP_SPEC_OPTIONS " }\n"
 		"",