docs(s3-presigned-post): fix to example in README (#2146)

`client` is a required first argument for `createPresignedPost`

Author: audionerd

packages/s3-presigned-post/README.md

@@ -46,7 +46,7 @@ const Key = "user/eric/1";
 const Fields = {
   acl: "public-read",
 };
-const { url, fields } = await createPresignedPost({
+const { url, fields } = await createPresignedPost(client, {
   Bucket,
   Key,
   Conditions,