Update README.md

Author: eliben

README.md

@@ -8,7 +8,7 @@ algorithm in Go. It's accompanied by a series of blog posts:
 * [Part 2: Commands and log replication](https://eli.thegreenplace.net/2020/implementing-raft-part-2-commands-and-log-replication/)
 * [Part 3: Persistence and optimizations](https://eli.thegreenplace.net/2020/implementing-raft-part-3-persistence-and-optimizations/)
 * [Part 4: Key/Value database](https://eli.thegreenplace.net/2024/implementing-raft-part-4-keyvalue-database/)
-* [Part 4: exactly-once delivery](https://eli.thegreenplace.net/2024/implementing-raft-part-5-exactly-once-delivery/)
+* [Part 5: exactly-once delivery](https://eli.thegreenplace.net/2024/implementing-raft-part-5-exactly-once-delivery/)
 
 Each of the `partN` directories in this repository is the complete source code
 for Part N of the blog post series (except Part 0, which is introductory and has