Fix wrong reference link in design docs.

[yanggangtony]

Thank you for contributing to Velero!

Please add a summary of your change

Like the pics , when i redirect the link , it is not right for me .

And I also search other place for fix this problem.
Just found 2 at present.
Maybe the user of design forget to update the place when he/she complete this feature, then the docs is moved to Implemented ,But the reference is not...

Does your change fix a particular issue?

Fixes #(issue)

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[yanggangtony]

/kind changelog-not-required

[codecov]

Codecov Report

Merging #7086 (7fd692e) into main (cb5ffe2) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #7086      +/-   ##
==========================================
- Coverage   61.16%   61.13%   -0.03%     
==========================================
  Files         256      256              
  Lines       27222    27222              
==========================================
- Hits        16650    16642       -8     
- Misses       9389     9396       +7     
- Partials     1183     1184       +1     

see 1 file with indirect coverage changes