Fix Amazon Kindle Embed Title

[kartikmehta8]

What?

Resolves: #68714

This PR addresses an issue in the Amazon Kindle Embed block where, after embedding a valid URL, the block incorrectly reverts to the default placeholder text instead of showing the correct text.

Why?

This PR is necessary to fix the user experience when embedding Kindle content. Currently, users expect the block to render the embedded preview after entering a valid URL, but the block erroneously displays the default placeholder text, creating confusion and reducing usability.

Testing Instructions

1. Open a post or page in the WordPress Gutenberg editor.
2. Add a new block by clicking the "+" button.
3. Search for and insert the Amazon Kindle Embed block.
4. Observe the default text in the block:
   • Expected: The block displays Amazon Kindle Embed - "Embed Amazon Kindle content."
5. Enter a valid Amazon Kindle URL in the URL field and press Enter.
6. Expected: The block displays Amazon Kindle Embed - "Embed Amazon Kindle content." [SAME]

Screenshots or screencast

Before	After

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