tools: bpftool: fix error message (prog -> object)

Change an error message to work for any object being
pinned not just programs.

Fixes: 71bb428f ("tools: bpf: add bpftool")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>