Update CHANGELOG.md

Author: jamescrosswell

CHANGELOG.md

@@ -1,11 +1,13 @@
 # Changelog
 
-## 5.1.1
+## Unreleased
 
 ### Features
 
 - The sample seed used for sampling decisions is now propagated, for use in downstream custom trace samplers ([#3951](https://github.com/getsentry/sentry-dotnet/pull/3951))
 
+## 5.1.1
+
 ### Fixes
 
 - Emit transaction.data inside contexts.trace.data ([#3936](https://github.com/getsentry/sentry-dotnet/pull/3936))