docs: Update 01-basic-markup.md with new svelte-ignore syntax

[shadow-identity]

01-basic-markup.md was using an old svelte-ignore syntax with dashed rule names (a11y-autofocus). I fixed it with a underscored one (a11y_autofocus).

There is no old-syntaxed svelte-ignore in the docs anymore.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 50090b6

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15394-svelte.vercel.app/

[shadow-identity]

(I don't know if my change should result in a new version 🤷🏼)

[Ocean-OS]

(I don't know if my change should result in a new version 🤷🏼)

If it doesn't change anything in packages/svelte, then it shouldn't result in a new version.

[Rich-Harris]

thank you!