docs/bpf: Fix malformed documentation

docs/bpf: Fix malformed documentation

Two issues are fixed:
  1. Malformed table due to newly-introduced BPF_MOVSX
  2. Missing reference link for ``Sign-extension load operations``

Fixes: 245d4c40c09b ("docs/bpf: Add documentation for new instructions")
Cc: bpf@ietf.org
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202307291840.Cqhj7uox-lkp@intel.com/
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230730004251.381307-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>