bpf, docs: Use SPDX license identifier in bpf_doc.py

The Linux man-pages project now uses SPDX tags, instead of the full
license text.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://www.kernel.org/doc/man-pages/licenses.html
Link: https://spdx.org/licenses/Linux-man-pages-copyleft.html
Link: https://lore.kernel.org/bpf/20220721110821.8240-1-alx.manpages@gmail.com