Electron README cleanup [ci skip]

use `XLSX` rather than `X` in README

demos/electron/README.md

@@ -24,7 +24,7 @@ var dialog = require('electron').remote.dialog;
 
 /* show a file-open dialog and read the first selected file */
 var o = dialog.showOpenDialog({ properties: ['openFile'] });
-var workbook = X.readFile(o[0]);
+var workbook = XLSX.readFile(o[0]);
 
 /* show a file-save dialog and write the workbook */
 var o = dialog.showSaveDialog();