Update rand_core changelog link (#1680)

Author: dhardy

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-A [separate changelog is kept for rand_core](rand_core/CHANGELOG.md).
+A [separate changelog is kept for rand_core](https://github.com/rust-random/core/blob/master/CHANGELOG.md).
 
 You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.html) useful.