fix: change parameter name for letter head

To match changes from https://github.com/frappe/frappe/pull/19627
diff --git a/erpnext/buying/doctype/request_for_quotation/request_for_quotation.js b/erpnext/buying/doctype/request_for_quotation/request_for_quotation.js
index a9f5afb..7f8dd56 100644
--- a/erpnext/buying/doctype/request_for_quotation/request_for_quotation.js
+++ b/erpnext/buying/doctype/request_for_quotation/request_for_quotation.js
@@ -129,7 +129,7 @@
 										supplier: data.supplier,
 										print_format: data.print_format || "Standard",
 										language: data.language || frappe.boot.lang,
-										letter_head: data.letter_head || frm.doc.letter_head || "",
+										letterhead: data.letter_head || frm.doc.letter_head || "",
 									}).toString()
 								)
 							);
diff --git a/erpnext/buying/doctype/request_for_quotation/request_for_quotation.py b/erpnext/buying/doctype/request_for_quotation/request_for_quotation.py
index 8e9ded9..332705e 100644
--- a/erpnext/buying/doctype/request_for_quotation/request_for_quotation.py
+++ b/erpnext/buying/doctype/request_for_quotation/request_for_quotation.py
@@ -388,7 +388,7 @@
 
 
 @frappe.whitelist()
-def get_pdf(doctype, name, supplier, print_format=None, language=None, letter_head=None):
+def get_pdf(doctype, name, supplier, print_format=None, language=None, letterhead=None):
 	# permissions get checked in `download_pdf`
 	if doc := get_rfq_doc(doctype, name, supplier):
 		download_pdf(
@@ -397,7 +397,7 @@
 			print_format,
 			doc=doc,
 			language=language,
-			letter_head=letter_head or None,
+			letterhead=letterhead or None,
 		)