docs: bpf: get rid of two warnings

Documentation/bpf/btf.rst:154: WARNING: Unexpected indentation.
Documentation/bpf/btf.rst:163: WARNING: Unexpected indentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>