doc: fix a typo in sample apps guide

doc: fix a typo in sample apps guide

This patch fixes a trivial typo in the sample apps guide.
commit 35b09d76f89e ("doc: use corelist instead of coremask") replaced
the usage of coremask (-c) with corelist (-l).
As a result of this patch, we have
./build/ipv4_multicast -l 0-3 -n 3 -- -p 0x3 -q 1
in the sample app guide, while the explanation immediately following
says:
In this command:
• The -c option enables cores 0, 1, 2 and 3

This patch fixes the
explanation to have "-l" instead of "-c".

Fixes: 35b09d76f89e ("doc: use corelist instead of coremask")
Cc: stable@dpdk.org
Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>