Second Anki add-on link broken in wiki

[imnasnainaec]

https://github.com/sillsdev/flashgrab/wiki#download--install mentions https://ankiweb.net/shared/info/915685712, which says

This shared item is missing or currently unavailable.

[rmunn]

https://ankiweb.net/shared/info/618450151 is still up, and links to this GitHub repo. We should perhaps contact the AnkiWeb owners to see about saying "Hey, the original owner of the account that uploaded the plugin is deceased, and we want to transfer the credentials to the organization". Then upload new versions of FlashGrab to https://ankiweb.net/shared/info/618450151 and update our wiki instructions accordingly.