MAGETWO-43907 Custom admin URL breaks admin log in after installation

- improved error message

Author: Ivan Gavryshko

setup/view/magento/setup/web-configuration.phtml

@@ -20,7 +20,7 @@ $hints = [
     ),
     'admin' => sprintf(
         '<p>%s</p>',
-        'A unique URL helps keep store and customer info safer. Use letters, numbers or underscores.'
+        'A unique URL helps keep store and customer info safer. Use only letters, numbers, and underscore characters.'
     )
 ];
 ?>
@@ -113,7 +113,7 @@ $hints = [
                         >
                     <div class="error-container">
                         <span ng-show="webconfig.admin.$error">
-                            Please enter a valid admin URL path. Use letters, numbers or underscores.
+                            Please enter a valid admin URL path. Use only letters, numbers, and underscore characters.
                         </span>
                     </div>
                 </div>