Update clean_redirects exclusion to new page

The page explaining redirects in the gitlab repo moved from doc/development/documentation/index.md to doc/development/documentation/redirects.md. We had to manually exclude this file because it's a false positive for this test. When it moved, it broke this Rake task. (Discovered in the 2021-09 monthly chores issue.)

Related to technical-writing#474 (closed) and specifically technical-writing#474 (comment 677598652)