Specify to use the commonly spelled out pagination

What does this MR do?

The API documentation for "List repository tree" only partially called out a single pagination parameter when there is already a common definition for pagination and page is a key part of that (not in the docs). This MR removes the per_page parameter call out and points that section to the overall pagination in the root (borrowed from audit events)

The existing documentation that I am looking at is here

Screenshots

N/A

Does this MR meet the acceptance criteria?

This should just be a documentation change (@gl-docsteam per here)

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