docs: update demo page css

Author: bennypowers

docs/demo/demo.css

@@ -10,14 +10,17 @@ body {
 html {
   min-height: 100%;
   display: grid;
-  column-gap: 1em;
   grid-template-rows: var(--pfe-demo-header-height) 1fr;
   grid-template-columns: var(--pfe-demo-menu-width) 1fr;
   grid-template-areas:
     'head head'
     'menu body';
 }
 
+#main-nav .site-navigation {
+  width: 100%;
+}
+
 header {
   grid-area: head;
   background: var(--pfe-theme--color--surface--accent, #004080);