bpf: typo fix in Documentation/networking/af_xdp.rst

Fix a simple typo: Completetion -> Completion

Signed-off-by: Konrad Djimeli <kdjimeli@igalia.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>