Functions changelog corrections (#6467)

Author: maksymmalyhin

Functions/CHANGELOG.md

@@ -1,6 +1,9 @@
-# v2.8.0
+# v2.9.0
 - [changed] Weak dependency on Instance ID replaced by Firebase Messaging. (#6395)
 
+# v2.8.0
+- [changed] New public header structure. (#6193)
+
 # v2.7.0
 - [changed] Functionally neutral source reorganization. (#5858)