Fix the following sphinx warning:

bpf_devel_QA.rst:444: WARNING: Unknown target name:
"documentation/bpf/btf.rst"

No target was defined for 'btf.rst' in the document. Fix it.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-6-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>