docs: update storefront development snippets for next.js 15 compatibility (#10843)

www/apps/resources/app/storefront-development/cart/update/page.mdx

@@ -76,7 +76,7 @@ fetch(`http://localhost:9000/store/carts/${cartId}/customer`, {
   method: "POST",
   headers: {
     "Content-Type": "application/json",
-    
+    "x-publishable-api-key": process.env.NEXT_PUBLIC_MEDUSA_PUBLISHABLE_KEY || "temp",
   },
 })
 .then((res) => res.json())