Correct focus-mode link anchor

Currently, the feature comparison link for Focus Mode goes here: https://docs.gitlab.com/ee/user/project/issue_board.html#focus-mode

The focus-mode anchor doesn't exist, so you're defaulted to the top of the page, which is confusing.

The Focus Mode section is anchored by focus-mode-starter which is actually here: https://docs.gitlab.com/ee/user/project/issue_board.html#focus-mode-starter