fix: Make Sales Order field editable so the PO can be linked with an SO later

erpnext/buying/doctype/purchase_order_item/purchase_order_item.json

@@ -488,7 +488,6 @@
    "no_copy": 1,
    "options": "Sales Order",
    "print_hide": 1,
-   "read_only": 1,
    "search_index": 1
   },
   {
@@ -836,7 +835,7 @@
  "index_web_pages_for_search": 1,
  "istable": 1,
  "links": [],
- "modified": "2021-06-28 19:22:22.715365",
+ "modified": "2021-08-27 01:35:37.755413",
  "modified_by": "Administrator",
  "module": "Buying",
  "name": "Purchase Order Item",