Fix installation command for MCP server

[OwainWilliams]

This pull request makes a minor update to the installation instructions in the README.md for the MCP developer reference. The change updates the global install command to reference the correct package.

• Updated the global installation example to use @umbraco-cms/mcp-dev@17 instead of @umbraco/mcp-server in the README.md documentation.Updated installation command for MCP server to use the correct package name.

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

[sofietoft]

Thanks for the PR @OwainWilliams , and Happy New Year!

I'll just run this by our developers for a quick check, before we merge it.