erp5: Jupyter internal service is subinstance, not slave instance

@vpelletier says:

    I do not think this is a "slave" instance, but just a sub-instance.

    "slave instance" is a way to pass parameter to a foreign instance. For example,
    frontend is an instance, and a slave instance is the rule given by an unrelated
    partition to make that existing frontend instance direct http://foo.com to 1.2.3.4:80 .

    OTOH, "balancer" is a sub-instance of ERP5's SR base instance (which is typically the root instance also).

/noticed-by @vpelletier (on nexedi/slapos!43)

software/erp5/instance-erp5-input-schema.json

@@ -237,10 +237,10 @@
       "type": "array"
     },
     "jupyter": {
-      "description": "Jupyter slave instance parameters",
+      "description": "Jupyter subinstance parameters",
       "properties": {
         "enable": {
-          "description": "Whether to enable creation of associated slave Jupyter instance",
+          "description": "Whether to enable creation of associated Jupyter subinstance",
           "default": false,
           "type": "boolean"
         },