Update database migration docs to avoid rubocop offense

What does this MR do?

After discussion about this here: !42978 (comment 429453619) it appears we don't need to use with_lock_retries in this example as the add_concurrent_foreign_key helper method already includes this functionality.

Update the docs to use the add_concurrent_foreign_key helper instead, so that we can avoid the need for disabling rubocop.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team