Fix ComputerCraftAPI.getWirelessNetwork() failing

I've got to admit, it is super embarrassing that a) I didn't notice this
when testing and b) no one else has noticed until now.

src/main/java/dan200/computercraft/ComputerCraft.java

@@ -766,7 +766,7 @@ public class ComputerCraft
         return upgrades;
     }
 
-    public IPacketNetwork getWirelessNetwork()
+    public static IPacketNetwork getWirelessNetwork()
     {
         return WirelessNetwork.getUniversal();
     }