net: core: dev.c: fix a documentation warning

There's a markup for link with is "foo_". On this kernel-doc
comment, we don't want this, but instead, place a literal
reference. So, escape the literal with ``foo``, in order to
avoid this warning:

	./net/core/dev.c:5195: WARNING: Unknown target name: "page_is".

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>