bpf, docs: Fix link to BTF doc

Ross reported broken link to BTF documentation
(Documentation/bpf/btf.rst) in Documentation/bpf/bpf_devel_QA.rst. The
link in question is written using external link syntax, with the target
refers to BTF doc in reST source (btf.rst), which doesn't exist in
resulting HTML output.

Fix the link by replacing external link syntax with simply writing out
the target doc, which the link will be generated to the correct HTML doc
target.

Fixes: 6736aa793c2b5f ("selftests/bpf: Add general instructions for test execution")
Reported-by: Ross Zwisler <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Ross Zwisler <[e-mail]>
Link: https://lore.kernel.org/linux-doc/[e-mail]/
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/bpf_devel_QA.rst

···
 tools/testing/selftests/bpf as closely as possible.
 
 Finally to ensure support for latest BPF Type Format features -
-discussed in `Documentation/bpf/btf.rst`_ - pahole version 1.16
+discussed in Documentation/bpf/btf.rst - pahole version 1.16
 is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
 pahole is delivered in the dwarves package or can be built
 from source at
···
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/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!