Fixed typo in slash commands docs

What does this MR do?

Typo fix (substract -> subtract)

Are there points in the code the reviewer needs to double check?

Nope

Why was this MR needed?

a typo

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added (N/A)
• Tests
  • Added for this feature/bug (N/A)
  • All builds are passing(N/A)
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27674 (closed)