[Vertex AI] Fix typo in 11.9.0 changelog and add link to guide (#14615)

Author: andrewheard

FirebaseVertexAI/CHANGELOG.md

@@ -11,12 +11,12 @@
   are required. (#14558)
 
 # 11.9.0
-- [feature] **Public Preview**: Added support for generating images using the
-  Imagen 3 model.
+- [feature] **Public Preview**: Added support for
+  [generating images](https://firebase.google.com/docs/vertex-ai/generate-images-imagen?platform=ios)
+  using the Imagen 3 models.
   <br /><br />
-  Note: This feature is in Public Preview, which means that the it is not
-  subject to any SLA or deprecation policy and could change in
-  backwards-incompatible ways.
+  Note: This feature is in Public Preview, which means that it is not subject to
+  any SLA or deprecation policy and could change in backwards-incompatible ways.
 - [feature] Added support for modality-based token count. (#14406)
 
 # 11.6.0