bpf: bpftool, fix documentation for attach types

bpftool has support for attach types "stream_verdict" and
"stream_parser" but the documentation was referring to them as
"skb_verdict" and "skb_parse". The inconsistency comes from commit
b7d3826c ("bpf: bpftool, add support for attaching programs to
maps").

This patch changes the documentation to match the implementation:
- "bpftool prog help"
- man pages
- bash completion

Signed-off-by: Alban Crequy <alban@kinvolk.io>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>