remove duplicate <h1> from native examples

[martymcguire]

Running the native example was producing invalid HTML with nested <h1>s:

<my-header enhanced="✨">  
    <h1><h1>Hello World</h1></h1></my-header>

my-header.php wraps the <slot> in an <h1> in both the native and WASM examples.

This PR updates the native $htmlString example in both the README and index.php to remove the extra <h1>.

[github-actions]

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[martymcguire]

I have read the CLA Document and I hereby sign the CLA