Add missing line in unreleased section for perf.

Author: rlazo

firebase-perf/CHANGELOG.md

@@ -1,8 +1,10 @@
 # Unreleased
+
 * [fixed] Fixed a bug where screen traces were not capturing frame metrics for multi-activity apps.
 * [feature] Added support for measuring screen performance metrics for "Activity Fragments" out-of-the-box.
 * [fixed] Excluded custom attributes whose key/value length is 0.
 
+
 # 20.2.0
 * [unchanged] Updated to accommodate the release of the updated
   [perfmon] Kotlin extensions library.