migration.md: fix Jekyll variable. (#24449)
migration.md: fix Jekyll variable. (#24449)

Rationale: the page is not available locally, only on gh-pages, thus leading to a 404 locally.
creativewebjp/bootstrap,bootstrapbrasil/bootstrap,vsn4ik/bootstrap,jipexu/bootstrap,realcrowd/bootstrap,Hemphill/bootstrap-docs,tibo66/bootstrap,creativewebjp/bootstrap,tjkohli/bootstrap,jipexu/bootstrap,fschumann1211/bootstrap,realcrowd/bootstrap,peterblazejewicz/bootstrap,tibo66/bootstrap,creativewebjp/bootstrap,tibo66/bootstrap,tagliala/bootstrap,tagliala/bootstrap,tjkohli/bootstrap,zalog/bootstrap,9-chirno/chirno,stanwmusic/bootstrap,bardiharborow/bootstrap,m5o/bootstrap,twbs/bootstrap,jipexu/bootstrap,zalog/bootstrap,bootstrapbrasil/bootstrap,gijsbotje/bootstrap,fschumann1211/bootstrap,janseliger/bootstrap,GerHobbelt/bootstrap,nice-fungal/bootstrap,janseliger/bootstrap,nice-fungal/bootstrap,supergibbs/bootstrap,inway/bootstrap,inway/bootstrap,stanwmusic/bootstrap,yuyokk/bootstrap,vsn4ik/bootstrap,m5o/bootstrap,bardiharborow/bootstrap,seanwu99/bootstrap,gijsbotje/bootstrap,janseliger/bootstrap,9-chirno/chirno,Lyricalz/bootstrap,bootstrapbrasil/bootstrap,seanwu99/bootstrap,twbs/bootstrap,gijsbotje/bootstrap,fschumann1211/bootstrap,supergibbs/bootstrap,coliff/bootstrap,SweetProcess/bootstrap,janseliger/bootstrap,joblocal/bootstrap,Lyricalz/bootstrap,realcrowd/bootstrap,fschumann1211/bootstrap,seanwu99/bootstrap,SweetProcess/bootstrap,Hemphill/bootstrap-docs,joblocal/bootstrap,peterblazejewicz/bootstrap,GerHobbelt/bootstrap,joblocal/bootstrap,Lyricalz/bootstrap,stanwmusic/bootstrap,bardiharborow/bootstrap,yuyokk/bootstrap,vsn4ik/bootstrap,SweetProcess/bootstrap,joblocal/bootstrap,vsn4ik/bootstrap,Hemphill/bootstrap-docs,creativewebjp/bootstrap,inway/bootstrap,gijsbotje/bootstrap,9-chirno/chirno,kvlsrg/bootstrap,9-chirno/chirno,jipexu/bootstrap,yuyokk/bootstrap,seanwu99/bootstrap,supergibbs/bootstrap,yuyokk/bootstrap,tibo66/bootstrap,realcrowd/bootstrap,bardiharborow/bootstrap,coliff/bootstrap,Lyricalz/bootstrap,GerHobbelt/bootstrap,supergibbs/bootstrap,SweetProcess/bootstrap,tagliala/bootstrap,kvlsrg/bootstrap,nice-fungal/bootstrap,kvlsrg/bootstrap,Hemphill/bootstrap-docs,stanwmusic/bootstrap,peterblazejewicz/bootstrap,zalog/bootstrap