CHANGELOG.md: add pr number

Author: dconeybe

firebase-firestore/CHANGELOG.md

@@ -9,7 +9,7 @@
 - [changed] Improve query performance via internal memoization of calculated document data.
   [#7370](//github.com/firebase/firebase-android-sdk/issues/7370)
 - [changed] Improve query performance by avoiding excessive Comparator instance creation.
-  [#NNNN](//github.com/firebase/firebase-android-sdk/issues/NNNN)
+  [#7388](//github.com/firebase/firebase-android-sdk/pull/7388)
 
 # 26.0.0