updates a migration example in the Getting Started guide [ci skip]
updates a migration example in the Getting Started guide [ci skip]

Closes #20705.

rails/rails,betesh/rails,MichaelSp/rails,Sen-Zhang/rails,travisofthenorth/rails,iainbeeston/rails,riseshia/railsguides.kr,pvalena/rails,alecspopa/rails,kachick/rails,kmcphillips/rails,kamipo/rails,tjschuck/rails,bogdanvlviv/rails,travisofthenorth/rails,BlakeWilliams/rails,illacceptanything/illacceptanything,richseviora/rails,kamipo/rails,esparta/rails,betesh/rails,illacceptanything/illacceptanything,jeremy/rails,BlakeWilliams/rails,lucasmazza/rails,Sen-Zhang/rails,koic/rails,rafaelfranca/omg-rails,printercu/rails,xlymian/rails,Spin42/rails,notapatch/rails,yasslab/railsguides.jp,elfassy/rails,riseshia/railsguides.kr,kmayer/rails,kirs/rails-1,xlymian/rails,tgxworld/rails,aditya-kapoor/rails,Stellenticket/rails,kenta-s/rails,arunagw/rails,mohitnatoo/rails,untidy-hair/rails,eileencodes/rails,esparta/rails,felipecvo/rails,MSP-Greg/rails,deraru/rails,georgeclaghorn/rails,richseviora/rails,gfvcastro/rails,arjes/rails,mathieujobin/reduced-rails-for-travis,iainbeeston/rails,untidy-hair/rails,lcreid/rails,Stellenticket/rails,yasslab/railsguides.jp,riseshia/railsguides.kr,brchristian/rails,travisofthenorth/rails,gfvcastro/rails,Envek/rails,EmmaB/rails-1,eileencodes/rails,pvalena/rails,repinel/rails,maicher/rails,mohitnatoo/rails,utilum/rails,rbhitchcock/rails,maicher/rails,printercu/rails,marklocklear/rails,Envek/rails,illacceptanything/illacceptanything,ngpestelos/rails,elfassy/rails,richseviora/rails,palkan/rails,yhirano55/rails,notapatch/rails,EmmaB/rails-1,rossta/rails,gauravtiwari/rails,mechanicles/rails,Sen-Zhang/rails,88rabbit/newRails,sergey-alekseev/rails,BlakeWilliams/rails,kmcphillips/rails,marklocklear/rails,illacceptanything/illacceptanything,georgeclaghorn/rails,fabianoleittes/rails,Erol/rails,yawboakye/rails,tgxworld/rails,joonyou/rails,koic/rails,arjes/rails,aditya-kapoor/rails,stefanmb/rails,coreyward/rails,kachick/rails,prathamesh-sonpatki/rails,assain/rails,yhirano55/rails,felipecvo/rails,utilum/rails,repinel/rails,sealocal/rails,flanger001/rails,tijwelch/rails,baerjam/rails,amoody2108/TechForJustice,palkan/rails,mtsmfm/rails,88rabbit/newRails,deraru/rails,vassilevsky/rails,Edouard-chin/rails,Erol/rails,repinel/rails,gavingmiller/rails,rafaelfranca/omg-rails,rbhitchcock/rails,pvalena/rails,vipulnsward/rails,shioyama/rails,ngpestelos/rails,aditya-kapoor/rails,yawboakye/rails,vassilevsky/rails,mathieujobin/reduced-rails-for-travis,bradleypriest/rails,betesh/rails,mtsmfm/rails,bradleypriest/rails,shioyama/rails,yalab/rails,Vasfed/rails,matrinox/rails,esparta/rails,joonyou/rails,untidy-hair/rails,kddeisz/rails,lcreid/rails,tijwelch/rails,lcreid/rails,illacceptanything/illacceptanything,flanger001/rails,gcourtemanche/rails,tjschuck/rails,kaspth/rails,felipecvo/rails,assain/rails,pvalena/rails,rossta/rails,rails/rails,eileencodes/rails,koic/rails,eileencodes/rails,rbhitchcock/rails,mijoharas/rails,prathamesh-sonpatki/rails,tgxworld/rails,88rabbit/newRails,bolek/rails,vipulnsward/rails,stefanmb/rails,printercu/rails,mijoharas/rails,flanger001/rails,kirs/rails-1,shioyama/rails,samphilipd/rails,yahonda/rails,prathamesh-sonpatki/rails,gfvcastro/rails,starknx/rails,tjschuck/rails,arunagw/rails,tjschuck/rails,vipulnsward/rails,yahonda/rails,kddeisz/rails,Spin42/rails,alecspopa/rails,Vasfed/rails,fabianoleittes/rails,bogdanvlviv/rails,mechanicles/rails,amoody2108/TechForJustice,vipulnsward/rails,Stellenticket/rails,ledestin/rails,samphilipd/rails,untidy-hair/rails,gcourtemanche/rails,schuetzm/rails,utilum/rails,hanystudy/rails,pschambacher/rails,kenta-s/rails,illacceptanything/illacceptanything,arunagw/rails,travisofthenorth/rails,mohitnatoo/rails,MichaelSp/rails,mechanicles/rails,voray/rails,mijoharas/rails,voray/rails,illacceptanything/illacceptanything,rossta/rails,mohitnatoo/rails,palkan/rails,MSP-Greg/rails,EmmaB/rails-1,Erol/rails,tgxworld/rails,kamipo/rails,yalab/rails,maicher/rails,mathieujobin/reduced-rails-for-travis,yawboakye/rails,baerjam/rails,repinel/rails,kenta-s/rails,rails/rails,gavingmiller/rails,illacceptanything/illacceptanything,lucasmazza/rails,Envek/rails,samphilipd/rails,gauravtiwari/rails,bogdanvlviv/rails,georgeclaghorn/rails,gfvcastro/rails,matrinox/rails,kmcphillips/rails,schuetzm/rails,odedniv/rails,kachick/rails,yahonda/rails,mtsmfm/railstest,MichaelSp/rails,lucasmazza/rails,MSP-Greg/rails,illacceptanything/illacceptanything,assain/rails,Edouard-chin/rails,matrinox/rails,Vasfed/rails,gavingmiller/rails,stefanmb/rails,pschambacher/rails,rafaelfranca/omg-rails,joonyou/rails,betesh/rails,jeremy/rails,notapatch/rails,aditya-kapoor/rails,baerjam/rails,kmayer/rails,xlymian/rails,Edouard-chin/rails,joonyou/rails,utilum/rails,odedniv/rails,kddeisz/rails,flanger001/rails,kmayer/rails,starknx/rails,yahonda/rails,arunagw/rails,kddeisz/rails,bradleypriest/rails,sealocal/rails,esparta/rails,ttanimichi/rails,iainbeeston/rails,elfassy/rails,coreyward/rails,kirs/rails-1,fabianoleittes/rails,ttanimichi/rails,yalab/rails,kaspth/rails,illacceptanything/illacceptanything,yhirano55/rails,pschambacher/ra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