doc: fix service core mask in l3fwd guide
doc: fix service core mask in l3fwd guide

Service core mask should be a hexadecimal value rather than a range of
lcores.

Bugzilla ID: 392
Fixes: 55499896d91a ("doc: add event mode to l3fwd guide")

Signed-off-by: Pavan Nikhilesh <8184a94f8f7778086ea0697cdb513b2e42813b09@marvell.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk