Fix typo in README
Fix typo in README

https://github.com/holdenk/spark-testing-base/issues/39

holdenk/spark-testing-base,snithish/spark-testing-base,snithish/spark-testing-base,snithish/spark-testing-base,mahmoudhanafy/spark-testing-base,snithish/spark-testing-base,mahmoudhanafy/spark-testing-base,holdenk/spark-testing-base,holdenk/spark-testing-base,mahmoudhanafy/spark-testing-base,holdenk/spark-testing-base,mahmoudhanafy/spark-testing-base