Fix number of classes in indic_glue sna.bn dataset
@lhoestq there are many things missing in the README.md file, but this correction is right despite not passing the validation tests... Yes indeed. We run the validation in all modified readme because we think that it is the time when contributors are the most likely to fix a dataset card - or it will never be done
As read in the [paper](https://www.aclweb.org/anthology/2020.findings-emnlp.445.pdf), Table 11.