docs: fix method names in complete checkout code block (#4746)

Closes #4744

docs/content/modules/carts-and-checkout/storefront/implement-checkout-flow.mdx

@@ -560,8 +560,8 @@ const Cart = () => {
   const { completeCheckout } = useCart()
   // ...
 
-  const handleStartCheckout = () => {
-    startCheckout.mutate()
+  const handleCompleteCheckout = () => {
+    completeCheckout.mutate()
   }
 
   // ...