Makefile: fix typo in helper message
Makefile: fix typo in helper message

Fixes: 2186ae4c3079 ("make: add help target to root Makefile for printing info about available targets")
Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium