docs: bpf/bpf_devel_QA.rst: fix reference to nonexistent document

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: NDaniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-6-dwlsalmeida@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/bpf/bpf_devel_QA.rst

@@ -643,5 +643,6 @@ when:
 .. _selftests: ../../tools/testing/selftests/bpf/
 .. _Documentation/dev-tools/kselftest.rst:
    https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
+.. _Documentation/bpf/btf.rst: btf.rst
 
 Happy BPF hacking!