libbpf: Fix malformed documentation formatting

This fixes the doxygen format documentation above the
user_ring_buffer__* APIs. There has to be a newline
before the @brief, otherwise doxygen won't render them
for libbpf.readthedocs.org.

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230126024749.522278-1-grantseltzer@gmail.com