Update purchase_taxes_and_charges.json label Rate to Tax Rate

Change Rate label to existing Tax Rate label so it can be correctly translated in other languages

(cherry picked from commit bd464197c4)

erpnext/accounts/doctype/purchase_taxes_and_charges/purchase_taxes_and_charges.json

@@ -126,7 +126,7 @@
    "fieldname": "rate",
    "fieldtype": "Float",
    "in_list_view": 1,
-   "label": "Rate",
+   "label": "Tax Rate",
    "oldfieldname": "rate",
    "oldfieldtype": "Currency"
   },
@@ -239,4 +239,4 @@
  "sort_field": "modified",
  "sort_order": "DESC",
  "track_changes": 1
-}
+}