Added 'var' to example to be strict mode compliant

closes #206

[ci skip]

README.md

@@ -238,7 +238,7 @@ This example iterates through every nonempty of every sheet and dumps values:
 var sheet_name_list = workbook.SheetNames;
 sheet_name_list.forEach(function(y) { /* iterate through sheets */
   var worksheet = workbook.Sheets[y];
-  for (z in worksheet) {
+  for (var z in worksheet) {
     /* all keys that do not begin with "!" correspond to cell addresses */
     if(z[0] === '!') continue;
     console.log(y + "!" + z + "=" + JSON.stringify(worksheet[z].v));