fix(buttons): change tiny button class to x-small (#1826)

Fixes button styling of tiny button on page
http://akveo.com/ngx-admin/#/pages/ui-features/buttons

src/app/pages/ui-features/buttons/size-buttons/size-buttons.component.html

@@ -37,13 +37,13 @@
     </div>
     <div class="size-container">
       <div class="container-title">
-        <span>Tiny Button</span>
+        <span>X-Small Button</span>
       </div>
       <div class="subheader">
         <span>0.5rem 1.25rem</span>
       </div>
       <div>
-        <button class="btn btn-primary btn-tn">Tiny</button>
+        <button class="btn btn-primary btn-xs">X-Small Button</button>
       </div>
     </div>
   </nb-card-body>