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 <danieltimlee@gmail.com>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20230811121603.17429-1-danieltimlee@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>