[Vertex AI] Update CHANGELOG entry with latest copy (#12947)

Author: andrewheard

FirebaseVertexAI/CHANGELOG.md

@@ -1,7 +1,6 @@
 # 10.26.0
 
-- [added] Public Preview release of the Vertex AI for Firebase SDK - build
-  AI-powered mobile apps and features with the Gemini API using this new SDK.
-  See the
-  [docs](https://firebase.google.com/docs/vertex-ai/get-started?platform=ios)
-  for instructions on how to get started.
+- [feature] Initial release of the Vertex AI for Firebase SDK (public preview).
+  Learn how to
+  [get started](https://firebase.google.com/docs/vertex-ai/get-started?platform=ios)
+  with the SDK in your app.