bpf, scripts: Correct GPL license name

The bpf_doc script refers to the GPL as the "GNU Privacy License".
I strongly suspect that the author wanted to refer to the GNU General
Public License, under which the Linux kernel is released, as, to the
best of my knowledge, there is no license named "GNU Privacy License".
This patch corrects the license name in the script accordingly.

Fixes: 56a092c8 ("bpf: add script and prepare bpf.h for new helpers documentation")
Signed-off-by: Gianmarco Lusvardi <glusvardi@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20240213230544.930018-3-glusvardi@posteo.net