fix(pos): variable typo: `s_pos` -> `is_pos`

Signed-off-by: Sabu Siyad <hello@ssiyad.com>
diff --git a/erpnext/public/js/controllers/taxes_and_totals.js b/erpnext/public/js/controllers/taxes_and_totals.js
index 7481000..1f8a5e3 100644
--- a/erpnext/public/js/controllers/taxes_and_totals.js
+++ b/erpnext/public/js/controllers/taxes_and_totals.js
@@ -58,7 +58,7 @@
 
 		if (
 			in_list(["Sales Invoice", "POS Invoice"], this.frm.doc.doctype)
-			&& this.frm.doc.s_pos
+			&& this.frm.doc.is_pos
 			&& this.frm.doc.is_return
 		) {
 			this.set_total_amount_to_default_mop();