Update widget-doughnut version to 1.5.6 in demo index.html

Author: Marko Petzold

demo/index.html

@@ -32,10 +32,10 @@
             const theme = { theme_name: 'vintage', theme_object: themeObject }
             render(
                 html`
-                    <widget-doughnut-1.5.5 .theme="${theme}" .inputData=${data}>
+                    <widget-doughnut-1.5.6 .theme="${theme}" .inputData=${data}>
                         web component is not registered. Make sure to use the tag with the correct version
                         string from the package.json
-                    </widget-doughnut-1.5.5>
+                    </widget-doughnut-1.5.6>
                 `,
                 document.getElementById('demo')
             )