Update reference to frontend guide in Dangerfile message

What does this MR do?

This MR updates a docs reference in the danger bot text.

Background:
In another MR - some developer documentation is being moved: !20010 (merged). The only reference to the moved docs in GitLab app code is in the Dangerfile. This MR updates the location to reference the new one.

Related Issue:

gitlab-org/frontend/general#25 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [n/a] Documentation created/updated or follow-up review issue created. (doc changes here)
• Code review guidelines
• [n/a] Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] Separation of EE specific content

Performance and Testing

• [n/a] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [n/a] Tested in all supported browsers

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:

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