Fix grammar in demo store notice

Author: erikhansen

app/code/Magento/Theme/view/frontend/templates/html/notices.phtml

@@ -54,7 +54,7 @@ require(['jquery'], function(jQuery){
 <?php if ($block->displayDemoNotice()): ?>
     <div class="message global demo">
         <div class="content">
-            <p><?= /* @escapeNotVerified */ __('This is demo store. No orders will be fulfilled.') ?></p>
+            <p><?= /* @escapeNotVerified */ __('This is a demo store. No orders will be fulfilled.') ?></p>
         </div>
     </div>
 <?php endif; ?>