[Vertex AI] Update README link to point to main (#1586)

Author: andrewheard

vertexai/README.md

@@ -1,5 +1,5 @@
 # Vertex AI for Firebase Quickstart
 
 Try out the
-[Vertex AI for Firebase Sample App](https://github.com/firebase/firebase-ios-sdk/tree/release-10.26/FirebaseVertexAI/Sample)
+[Vertex AI for Firebase Sample App](https://github.com/firebase/firebase-ios-sdk/tree/main/FirebaseVertexAI/Sample)
 in the `firebase-ios-sdk` repository to get started.