Remove invalid code block language tag in sitemap README

[delucis]

Changes

• Removes use of txt as a language tag in a code block, which is not valid and causes error logging in docs.
• Had to use a <!-- prettier-ignore --> comment because otherwise Prettier tries to remove the whitespace between the opening backticks and the metadata docs uses for insertion highlighting and filename.
• No changeset because in non-docs contexts this change is essentially invisible.

Testing

Docs-only, n/a.

Docs

/cc @withastro/maintainers-docs for feedback!

[changeset-bot]

⚠️ No Changeset found

Latest commit: d6322ba

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[ElianCodes]

Weird that prettier removes the whitespace, but this looks like a valid workaround to me!

[delucis]

Thanks for reviewing!

I’m guessing Prettier’s assumption is that the whitespace is a typo between the backticks and the language tag (because our syntax is admittedly nonstandard) and thinks it’s fixing that?