ElasticSearch Integration: fix class name in search results

What does this MR do?

According to Troubleshooting Elasticsearch, the class name of the search results with Elasticsearch enabled is Kaminari::PaginatableArray.

Our own gitlab-ee instance shows the same behaviour.

This MR updates the doc to use the correct class name.

Screenshots

No screenshots.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry: no, updating docs.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Change does affect only the documentation.

• 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

No security related changes.

• 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