Fix PR number in changelog for #1021 (#1064)

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

CHANGELOG.md

@@ -12,7 +12,7 @@ New:
 - Clarify env variables in otlp exporter
   ([#975](https://github.com/open-telemetry/opentelemetry-specification/pull/975))
 - Add Prometheus exporter environment variables
-  ([#2021](https://github.com/open-telemetry/opentelemetry-specification/pull/1021)).
+  ([#1021](https://github.com/open-telemetry/opentelemetry-specification/pull/1021)).
 - Default propagators in un-configured API must be no-op
   ([#930](https://github.com/open-telemetry/opentelemetry-specification/pull/930)).
 - Define resource mapping for Jaeger exporters