tools: bpftool: fix man page documentation for "pinmaps" keyword

The "pinmaps" keyword is present in the man page, in the verbose
description of the "bpftool prog load" command. However, it is missing
from the summary of available commands at the beginning of the file. Add
it there as well.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>