nixos-containers: fix dummyConfig localMacAddress default

In a13b423085 ("nixos-containers: allow hard-coding container veth MAC address &
add IPv6 SLAAC test case (#462775)") I had a typo in the `localMacAddress`
setting used for the dummy container config to generate the systemd service
template. I don't think this actually causes any breakage, but it's good to fix
for consistency's sake nonetheless.

nixos/modules/virtualisation/nixos-containers.nix

@@ -520,7 +520,7 @@ let
     hostAddress6 = null;
     localAddress = null;
     localAddress6 = null;
-    localmacAddress = null;
+    localMacAddress = null;
     tmpfs = null;
   };