Fix loader url composition on image/next doc (vercel#25893)

Remove the extra slash to avoid double slashes on returned url (the image src prop needs to start with a slash)

docs/api-reference/next/image.md

@@ -132,7 +132,7 @@ const MyImage = (props) => {
   return (
     <Image
       loader={myLoader}
-      src="/me.png"
+      src="me.png"
       alt="Picture of the author"
       width={500}
       height={500}