Update 03-forms-and-mutations.mdx (vercel#55386)

A unique key is required for each item on a list.

Author: alexandreferreirafr

docs/02-app/01-building-your-application/02-data-fetching/03-forms-and-mutations.mdx

@@ -702,8 +702,8 @@ export function Thread({ messages }: { messages: Message[] }) {
 
   return (
     <div>
-      {optimisticMessages.map((m) => (
-        <div>{m.message}</div>
+      {optimisticMessages.map((m, k) => (
+        <div key={k}>{m.message}</div>
       ))}
       <form
         action={async (formData: FormData) => {