Fix old reference to _sheetIndex in HTML Writer

Author: MarkBaker

Classes/PHPExcel/Writer/HTML.php

@@ -398,7 +398,7 @@ public function generateSheetData()
         }
 
         // Ensure that Spans have been calculated?
-        if ($this->_sheetIndex !== null || !$this->spansAreCalculated) {
+        if ($this->sheetIndex !== null || !$this->spansAreCalculated) {
             $this->calculateSpans();
         }