Update README.org

Reading XLSX file demo gave an error like this.
"multiple-value cell.String() in single-value context"

README.org

@@ -40,7 +40,7 @@ func main() {
     for _, sheet := range xlFile.Sheets {
         for _, row := range sheet.Rows {
             for _, cell := range row.Cells {
-                text := cell.String()
+                text, _ := cell.String()
                 fmt.Printf("%s\n", text)
             }
         }