doc: fix warnings

ryu/lib/packet/icmpv6.py:docstring of ryu.lib.packet.icmpv6.nd_option_pi:23: WARNING: Inline emphasis start-string without end-string.
ryu/doc/source/ofproto_v1_2_ref.rst:: WARNING: document isn't included in any toctree
ryu/doc/source/ofproto_v1_3_ref.rst:: WARNING: document isn't included in any toctree

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>