doc: index.html: fix aria-hiddentypo

fix typo in attribute; add a missing one.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/index.html

@@ -544,7 +544,7 @@
             </li>
             <li>
               <a href="project/release_process.html#long-term-support-lts" class="persona-link">
-                <i class="fa fa-chevron-right aria-hidden=" true""></i>
+                <i class="fa fa-chevron-right" aria-hidden="true"></i>
                 <span>Long-term Support</span>
               </a>
             </li>
@@ -577,7 +577,7 @@
       </div>
       <div class="cta-text">
         <span>Read the Guidelines</span>
-        <i class="fa fa-arrow-right"></i>
+        <i class="fa fa-arrow-right" aria-hidden="true"></i>
       </div>
     </a>
   </section>