CHANGELOG.md: add pr number

Author: dconeybe

firebase-firestore/CHANGELOG.md

@@ -8,8 +8,8 @@
   [#7376](//github.com/firebase/firebase-android-sdk/issues/7376)
 - [changed] Improve query performance via internal memoization of calculated document data.
   [#7370](//github.com/firebase/firebase-android-sdk/issues/7370)
-- [changed] Switch TreeSet to HashSet to improve query execution performance.
-  [#NNNN](//github.com/firebase/firebase-android-sdk/issues/NNNN)
+- [changed] Improve query performance by using an unsorted HashMap instead of a sorted TreeMap.
+  [#7389](//github.com/firebase/firebase-android-sdk/pull/7389)
 
 # 26.0.0