docs: networking: Fix failover build warnings

Currently building the net_failover docs causes a bunch of warnings to
be emitted.  These warnings are all related to indentation and correctly
highlight missing '::' (for code sections).  It looks, from other rst
files in Documentation, that the first column should be indented 2
spaces.

Add '::' before code snippets and indent all snippets uniformly starting
with 2 spaces.

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>