fix typo in readme
Dullard::Sheet now find the corresponding xl/worksheets/sheet#.xml file based on the index position in xl/workbook.xml instead of the sheetId.
- This situation came about when I had a workbook with two sheets (sheetId 9 and 8) with corresponding xml files named sheet1.xml and sheet2.xml.
Merge pull request #1 from c0/master

Changes to Dullard::Sheet xml file reference