docs: return json from the controller examples.

Author: thruflo

README.md

@@ -189,7 +189,7 @@ defmodule MutationController do
       )
       |> Writer.apply(transaction, Repo, format: Format.TanstackDB)
 
-    render(conn, :mutations, txid: txid)
+    json(conn, txid: txid)
   end
 end
 ```

lib/phoenix/sync/writer.ex

@@ -51,7 +51,7 @@ defmodule Phoenix.Sync.Writer do
               format: Format.TanstackDB
             )
 
-          render(conn, :mutations, txid: txid)
+          json(conn, txid: txid)
         end
       end