Update CHANGELOG.md

Author: milaGGL

firebase-firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+* [fixed] Fixed an issue caused by calling mutation on immutable map object. [#5573](//github.com/firebase/firebase-android-sdk/pull/5573)
+
 * [fixed] Fixed an issue in the local cache synchronization logic where all locally-cached documents that matched a resumed query would be unnecessarily re-downloaded; with the fix it now only downloads the documents that are known to be out-of-sync. [#5506](//github.com/firebase/firebase-android-sdk/pull/5506)