Fix minor error in the mentioned section name

What does this MR do?

It fixes a minor error on the User email confirmation at sign-up documentation page. The mentioned Send confirmation email on sign-up option is available under the Sign-up Restrictions section, rather than the Sign-in Restrictions section.

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

No.

Why was this MR needed?

To save people a very small amount of time when they're looking for the mentioned option.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?