fix socket argument to properly reflect default

The scanner uses two different Unix sockets, and the existing ExecStart line appears to be confusing the two. The --unix-socket command line parameter refers to the management socket that the manager uses to connect to the scanner. Its default value is the one I have specified in the patch.

The scanner uses a separate socket to connect to a Redis server to access what OpenVAS calls the "KB," which apparently stands for "knowledge base." There is stored the NVT cache. There is no way to specify that socket path on the command line. It must be put in a config file.