[SQL] Correct a variable name in JavaApplySchemaSuite.applySchemaToJSON

`schemaRDD2` is not tested because `schemaRDD1` is registered again.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #2869 from yhuai/JavaApplySchemaSuite and squashes the following commits:

95fe894 [Yin Huai] Correct variable name.