Fix example in BooleanWidget docs (#1276)
Fix example in BooleanWidget docs (#1276)

- Fix wrong function name `before_row_import` to 
`before_import_row` in BooleanWidget's example
django-import-export/django-import-export,jnns/django-import-export,jnns/django-import-export,django-import-export/django-import-export,django-import-export/django-import-export,jnns/django-import-export,jnns/django-import-export,django-import-export/django-import-export