bpf, xdp, docs: Correct some English grammar and spelling

Header DOC on include/net/xdp.h contained a few English grammer and
spelling errors.

Signed-off-by: Kev Jackson <foamdino@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/YVVaWmKqA8l9Tm4J@kev-VirtualBox