Documentation/networking: fix af_xdp.rst Sphinx warnings

Fix Sphinx warnings in Documentation/networking/af_xdp.rst by
adding indentation:

Documentation/networking/af_xdp.rst:319: WARNING: Literal block expected; none found.
Documentation/networking/af_xdp.rst:326: WARNING: Literal block expected; none found.

Fixes: 0f4a9b7d ("xsk: add FAQ to facilitate for first time users")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Magnus Karlsson <magnus.karlsson@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>