Fix cell location error in validation example.
Fix cell location error in validation example.

Closes #314

jkyeung/XlsxWriter,jkyeung/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter