Update CHANGELOG

Author: utpilla

src/OpenTelemetry/CHANGELOG.md

@@ -5,6 +5,9 @@
 * Increased the character limit of the Meter instrument name from 63 to 255.
   ([#4774](https://github.com/open-telemetry/opentelemetry-dotnet/issues/4774))
 
+* Update default size for `SimpleExemplarReservoir` to `1`.
+  ([#4803](https://github.com/open-telemetry/opentelemetry-dotnet/issues/4803))
+
 ## 1.6.0-rc.1
 
 Released 2023-Aug-21