bpftool: Fix typo in bash-completion

_bpftool_get_map_names => _bpftool_get_prog_names for prog-attach|detach.

Fixes: 99f9863a

 ("bpftool: Match maps by name")
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200309173218.2739965-5-songliubraving@fb.com