Fix migration in README.md
Fix migration in README.md

resolves #297 https://github.com/collectiveidea/awesome_nested_set/issues/297

Closes #301

[ci skip]

markmercedes/awesome_nested_set,collectiveidea/awesome_nested_set,sximba/rational_nested_set,experteer/awesome_nested_set,rbo7nik/awesome_nested_set,oesgalha/awesome_nested_set,sximba/rational_nested_set,petergoldstein/awesome_nested_set