specify that hex strings must be lowercase for hashing.

nips/01.md

@@ -35,7 +35,7 @@ To obtain the `event.id`, we `sha256` the serialized event. The serialization is
 ```
 [
   0,
-  <pubkey, as a hex string>,
+  <pubkey, as a (lowercase) hex string>,
   <created_at, as a number>,
   <kind, as a number>,
   <tags, as an array of arrays>,