net: shell: Update info to enable allocs

If relevant configs are not enabled shell allocs command would print
wrong info:

"Enable CONFIG_NET_DEBUG_NET_PKT to see allocations"

CONFIG_NET_DEBUG_NET_PKT is obsolete and an undefined symbol, setting it
causes cmake to fail. This patch fixes this issue by printing up-to-date
information on how to enable allocs command

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>