bpf, doc: Update Andrii's email in MAINTAINERS

Update Andrii Nakryiko's reviewer email to kernel.org account. This optimizes
email logistics on my side and makes it less likely for me to miss important
patches.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201005223648.2437130-1-andrii@kernel.org