libbpf: update index.rst reference

Changeset d20b41115a ("libbpf: Rename libbpf documentation index file")
renamed: Documentation/bpf/libbpf/libbpf.rst
to: Documentation/bpf/libbpf/index.rst.

Update its cross-reference accordingly.

Fixes: d20b41115a ("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>

Documentation/bpf/index.rst

@@ -15,7 +15,7 @@ that goes into great technical depth about the BPF Architecture.
 libbpf
 ======
 
-Documentation/bpf/libbpf/libbpf.rst is a userspace library for loading and interacting with bpf programs.
+Documentation/bpf/libbpf/index.rst is a userspace library for loading and interacting with bpf programs.
 
 BPF Type Format (BTF)
 =====================