Fix multiple H1 headings in client-vs-server-rendering.md

Changed second H1 to H2 for proper document structure:
- Line 21: "# Different Server-Side Rendering Code" → "## Different..."

Having multiple H1s in a document is problematic for:
- SEO (search engines expect one H1 per page)
- Accessibility (screen readers use heading hierarchy)
- Documentation systems (assume H1 is the page title)
- Navigation/table of contents generation

The second heading is a subtopic about techniques for having different
client/server code, so it should be H2 under the main page topic.

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

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

Author: ihabadham

docs/core-concepts/client-vs-server-rendering.md

@@ -18,7 +18,7 @@ If you open the HTML source of any web page using React on Rails, you'll see the
 
 **Note**: If server rendering is not used (prerender: false), then the major difference is that the HTML rendered for the React component only contains the outer div: `<div id="HelloWorld-react-component-0"/>`. The first specification of the React component is just the same.
 
-# Different Server-Side Rendering Code (and a Server-Specific Bundle)
+## Different Server-Side Rendering Code (and a Server-Specific Bundle)
 
 You may want different code for your server-rendered components running server-side versus client-side. For example, if you have an animation that runs when a component is displayed, you might need to turn that off when server rendering. One way to handle this is conditional code like `if (window) { doClientOnlyCode() }`.