docs: networking: arcnet-hardware.rst: don't duplicate chapter names

docs: networking: arcnet-hardware.rst: don't duplicate chapter names

Since changeset 79f4fc61ad95 ("docs: fix reference to core-api/namespaces.rst"),
auto-references for chapters are generated. This is a nice feature, but
has a drawback: no chapters can have the same sumber.

So, we need to change two chapter titles, to avoid warnings when
building the docs.

Fixes: 79f4fc61ad95 ("docs: fix reference to core-api/namespaces.rst")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>