Update example of Server Actions HoC (#51299)

Currently the Server Action function with `"use server"` must be an
async function as it's required by the compiler, even if it returns a
promise already.

docs/02-app/01-building-your-application/03-data-fetching/04-server-actions.mdx

@@ -480,7 +480,7 @@ export const action = withValidate((data) => {
 
 ```js filename="lib/form-validation.js"
 export function withValidate(action) {
-  return (formData: FormData) => {
+  return async (formData: FormData) => {
     'use server'
 
     const isValidData = verifyData(formData)