Revert a11y change

Author: OS-pedrolourenco

core/src/components/tab-button/test/a11y/index.html

@@ -15,7 +15,7 @@
     <main>
       <h1>Tab Button</h1>
       <ion-tab-bar>
-        <ion-tab-button href="#" shape="rectangular">
+        <ion-tab-button href="#">
           <ion-label>Tab One</ion-label>
           <ion-icon aria-hidden="true" name="star"></ion-icon>
         </ion-tab-button>