bpf, doc: Do not link to docs.kernel.org for kselftest link

The question on how to run BPF selftests have a reference link to kernel
selftest documentation (Documentation/dev-tools/kselftest.rst). However,
it uses external link to the documentation at kernel.org/docs (aka
docs.kernel.org) instead, which requires Internet access.

Fix this and replace the link with internal linking, by using :doc: directive
while keeping the anchor text.

Fixes: b7a27c3a ("bpf, doc: howto use/run the BPF selftests")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230228074523.11493-2-bagasdotme@gmail.com