migration.md: a few grammar fixes
migration.md: a few grammar fixes

GerHobbelt/bootstrap,m5o/bootstrap,zalog/bootstrap,GerHobbelt/bootstrap,twbs/bootstrap,twbs/bootstrap,inway/bootstrap,m5o/bootstrap,tjkohli/bootstrap,tjkohli/bootstrap,inway/bootstrap,inway/bootstrap,zalog/bootstrap,zalog/bootstrap,GerHobbelt/bootstrap,coliff/bootstrap,coliff/bootstrap