bpf, doc: Update KP's email in MAINTAINERS

Helps me use a single account to sign off and send patches use
appropriate email redirection without needing to update MAINTAINERS.

Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201208214900.80684-1-kpsingh@kernel.org