docs/bpf: Update btf selftests program and add link

Commit c64779e2("selftests/bpf: Merge most of test_btf into test_progs")
renamed the BTF selftest from 'test_btf.c' to 'prog_tests/btf.c'.
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/tencent_1FA6904156E8E599CAE4ABDBE80F22830106@qq.com

Documentation/bpf/btf.rst

@@ -1062,4 +1062,9 @@ format.::
 7. Testing
 ==========
 
-Kernel bpf selftest `test_btf.c` provides extensive set of BTF-related tests.
+The kernel BPF selftest `tools/testing/selftests/bpf/prog_tests/btf.c`_
+provides an extensive set of BTF-related tests.
+
+.. Links
+.. _tools/testing/selftests/bpf/prog_tests/btf.c:
+   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/btf.c