docs: fix astro loader utils installation instruction

[HiDeoo]

This PR updates the README.md file for the Astro loader utils and specifically the installation command:

• Fix the package name: @ascorbic/feed-utils → @ascorbic/loader-utils
• Removes the astro@experimental--contentlayer experimental release (I think it's fine at this point?)

[changeset-bot]

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Latest commit: 8751015

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[ascorbic]

Thanks!