Cosmetic fix in doc/user/markdown.md where the word 'itself' is repeated.

What does this MR do?

(Small) Fix where the word 'itself' is repeated in doc/user/markdown.md

Does this MR meet the acceptance criteria?

Should do - it's really small. Not sure if it needs any of the detailed steps below.

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

Low risk. Single-word doc change.

Security

N/A