fix a code sample in README

- add pool.close() usage

README.md

@@ -160,6 +160,8 @@ let relaysForEvent = pool.seenOn(
   '44e1827635450ebb3c5a7d12c1f8e7b2b514439ac10a67eef3d9fd9c5c68e245'
 )
 // relaysForEvent will be an array of URLs from relays a given event was seen on
+
+pool.close()
 ```
 
 ### Parsing references (mentions) from a content using NIP-10 and NIP-27