ENGCOM-621: Fix grammar in demo store notice #13877

Author: Oleksii Korshenko

app/code/Magento/Theme/i18n/en_US.csv

@@ -107,7 +107,7 @@ Remove,Remove
 "For the best experience on our site, be sure to turn on Javascript in your browser.","For the best experience on our site, be sure to turn on Javascript in your browser."
 "Local Storage seems to be disabled in your browser.","Local Storage seems to be disabled in your browser."
 "For the best experience on our site, be sure to turn on Local Storage in your browser.","For the best experience on our site, be sure to turn on Local Storage in your browser."
-"This is demo store. No orders will be fulfilled.","This is demo store. No orders will be fulfilled."
+"This is a demo store. No orders will be fulfilled.","This is a demo store. No orders will be fulfilled."
 "Items %1 to %2 of %3 total","Items %1 to %2 of %3 total"
 "%1 Item","%1 Item"
 "%1 Item(s)","%1 Item(s)"

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; ?>