Fix minor spelling error in README
Fix minor spelling error in README

Alternativly -> Alternatively

Author: Guillaume Simard <guisim@gmail.com>

Closes #145 from GuiSim/patch-1.

databricks/spark-avro,CrazyJacky/spark-avro,CrazyJacky/spark-avro,tresata-opensource/spark-avro,tresata-opensource/spark-avro,databricks/spark-avro