Update terminology: Early hydration -> Immediate hydration

To align with PR #1997 which removes the immediate_hydration config and
makes it automatic for Pro users, updated the README to use the correct
terminology "Immediate hydration" instead of "Early hydration".

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails_pro/README.md

@@ -494,7 +494,7 @@ A: Pro adds performance features on top of the open-source gem:
 - Proper Node.js rendering pool
 - React Server Components
 - Streaming SSR
-- Early hydration
+- Immediate hydration
 - Code splitting with SSR support
 
 [See full comparison →](#pro-vs-open-source)