Minor styles nesting fix

Author: rogyar

app/design/frontend/Magento/luma/Magento_Checkout/web/css/source/module/checkout/_payments.less

@@ -58,10 +58,9 @@
                     }
                 }
             }
-            .form-purchase-order {
-                #po_number {
-                    margin-bottom: 20px;
-                }
+            
+            #po_number {
+                margin-bottom: 20px;
             }
         }