bpf: btf: fix the brackets of BTF_INT_OFFSET()

'VAL' should be protected by the brackets.

v2:
* Squash the fix for Documentation/bpf/btf.rst

Fixes: 69b693f0

 ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Gary Lin <glin@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>