demo: fix PDF export (#914)

fix #914

BackendType import wasn't correct (though the linter didn't give a warning)

demo/index.js

@@ -1,4 +1,5 @@
-import { OpenSheetMusicDisplay, BackendType } from '../src/OpenSheetMusicDisplay/OpenSheetMusicDisplay';
+import { OpenSheetMusicDisplay } from '../src/OpenSheetMusicDisplay/OpenSheetMusicDisplay';
+import { BackendType } from '../src/OpenSheetMusicDisplay/OSMDOptions';
 import * as jsPDF  from '../node_modules/jspdf-yworks/dist/jspdf.min';
 import * as svg2pdf from '../node_modules/svg2pdf.js/dist/svg2pdf.min';
 // import { Fraction } from '../src/Common/DataObjects/Fraction';