Admin#project-v12: fix broken hyperlink subaddress

[eugenepeh]

In https://github.com/nus-cs2103/website-base/blob/master/admin/project-v12.md, the hyperlink sub-address that directs to undo/redo feature implementation, in the AddressBook-Level4 developer guide, is broken.

Let's fix it by replacing the sub-address, #undo-redo-mechanism, with #undo-redo-feature.

[damithc]

Thanks @eugenepeh Is the link broken in the current website? In that case, also submit a PR for this sem's repo because we are not syncing the semester repo with the base repo during the semester.

[eugenepeh]

Done, did an unclean quick PR for this sem's repo.