libbpf: update index.rst reference

libbpf: update index.rst reference

Changeset d20b41115ad5 ("libbpf: Rename libbpf documentation index file")
renamed: Documentation/bpf/libbpf/libbpf.rst
to: Documentation/bpf/libbpf/index.rst.

Update its cross-reference accordingly.

Fixes: d20b41115ad5 ("libbpf: Rename libbpf documentation index file")
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>