fix: Outstanding Amount field of Discounted Invoices Table in Invoice Discounting

It wrongly fetched grand total from sales invoice instead of outstanding amount.
diff --git a/erpnext/accounts/doctype/discounted_invoice/discounted_invoice.json b/erpnext/accounts/doctype/discounted_invoice/discounted_invoice.json
index 04d6303..ee7f750 100644
--- a/erpnext/accounts/doctype/discounted_invoice/discounted_invoice.json
+++ b/erpnext/accounts/doctype/discounted_invoice/discounted_invoice.json
@@ -38,7 +38,7 @@
    "read_only": 1
   },
   {
-   "fetch_from": "sales_invoice.grand_total",
+   "fetch_from": "sales_invoice.outstanding_amount",
    "fieldname": "outstanding_amount",
    "fieldtype": "Currency",
    "in_list_view": 1,
@@ -60,7 +60,7 @@
   }
  ],
  "istable": 1,
- "modified": "2019-08-07 15:13:55.808349",
+ "modified": "2019-09-23 13:59:16.450344",
  "modified_by": "Administrator",
  "module": "Accounts",
  "name": "Discounted Invoice",