Fix code referencing wrong class (#12263)
Fix code referencing wrong class (#12263)

For some reason, I have seen this only triggered here:
https://circleci.com/gh/tootsuite/mastodon/98324?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-checks-link

But Follow.find_by referenced the ActivityPub::Activity::Follow class
instead of the model class.
Ryanaka/mastodon,pinfort/mastodon,masto-donte-com-br/mastodon,h3zjp/mastodon,musashino205/mastodon,maa123/mastodon,primenumber/mastodon,theoria24/mastodon,dunn/mastodon,tootcafe/mastodon,musashino205/mastodon,glitch-soc/mastodon,masto-donte-com-br/mastodon,lynlynlynx/mastodon,cobodo/mastodon,Nyoho/mastodon,lindwurm/mastodon,cobodo/mastodon,Ryanaka/mastodon,danhunsaker/mastodon,gol-cha/mastodon,ikuradon/mastodon,kirakiratter/mastodon,tootsuite/mastodon,MitarashiDango/mastodon,koba-lab/mastodon,dunn/mastodon,primenumber/mastodon,Ryanaka/mastodon,Kirishima21/mastodon,tootcafe/mastodon,imas/mastodon,tri-star/mastodon,abcang/mastodon,MitarashiDango/mastodon,im-in-space/mastodon,kirakiratter/mastodon,yi0713/mastodon,theoria24/mastodon,Ryanaka/mastodon,yukimochi/mastodon,dunn/mastodon,glitch-soc/mastodon,primenumber/mastodon,theoria24/mastodon,cobodo/mastodon,Nyoho/mastodon,maa123/mastodon,musashino205/mastodon,maa123/mastodon,abcang/mastodon,ykzts/mastodon,lynlynlynx/mastodon,koba-lab/mastodon,imas/mastodon,tri-star/mastodon,ykzts/mastodon,abcang/mastodon,koba-lab/mastodon,lindwurm/mastodon,yukimochi/mastodon,maa123/mastodon,MitarashiDango/mastodon,Nyoho/mastodon,lynlynlynx/mastodon,h3zjp/mastodon,masto-donte-com-br/mastodon,tootcafe/mastodon,yukimochi/mastodon,danhunsaker/mastodon,Kirishima21/mastodon,kirakiratter/mastodon,kazh98/social.arnip.org,yi0713/mastodon,danhunsaker/mastodon,ikuradon/mastodon,Kirishima21/mastodon,primenumber/mastodon,tri-star/mastodon,im-in-space/mastodon,rainyday/mastodon,rainyday/mastodon,lindwurm/mastodon,ikuradon/mastodon,ykzts/mastodon,Kirishima21/mastodon,gol-cha/mastodon,koba-lab/mastodon,pinfort/mastodon,glitch-soc/mastodon,kazh98/social.arnip.org,ikuradon/mastodon,dunn/mastodon,yi0713/mastodon,MitarashiDango/mastodon,rainyday/mastodon,tootsuite/mastodon,Nyoho/mastodon,glitch-soc/mastodon,gol-cha/mastodon,imas/mastodon,tootcafe/mastodon,rainyday/mastodon,h3zjp/mastodon,masto-donte-com-br/mastodon,im-in-space/mastodon,tri-star/mastodon,kazh98/social.arnip.org,tootsuite/mastodon,yi0713/mastodon,imas/mastodon,lynlynlynx/mastodon,kazh98/social.arnip.org,abcang/mastodon,pinfort/mastodon,pinfort/mastodon,cobodo/mastodon,musashino205/mastodon,danhunsaker/mastodon,tootsuite/mastodon,gol-cha/mastodon,ykzts/mastodon,kirakiratter/mastodon,h3zjp/mastodon,yukimochi/mastodon,lindwurm/mastodon,theoria24/mastodon,im-in-space/mastodon