fix minor typo on nip-01

nips/01.md

@@ -39,7 +39,7 @@ To obtain the `event.id`, we `sha256` the serialized event. The serialization is
   <created_at, as a number>,
   <kind, as a number>,
   <tags, as an array of arrays>,
-  <content>, as a string
+  <content, as a string>
 ]
 ```