Better MDN reference (#13144)

docs/api/fetch.md

@@ -42,7 +42,7 @@ const response = await fetch("http://example.com", {
 });
 ```
 
-`body` can be a string, a `FormData` object, an `ArrayBuffer`, a `Blob`, and more. See the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/API/Body/body) for more information.
+`body` can be a string, a `FormData` object, an `ArrayBuffer`, a `Blob`, and more. See the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#setting_a_body) for more information.
 
 ### Proxying requests