doc: fix angular logo at welcome page

Author: l-lin

demo/src/app/welcome.component.html

@@ -13,7 +13,7 @@ <h1 class="header center-on-small-only">Angular DataTables</h1>
     <div class="col s12 m9 l12">
       <div class="section">
         <p class="caption">
-          <a href="https://angular.io/"><img src="https://angular.io/resources/images/logos/angular/angular.svg" height="32px" />&nbsp;Angular</a>
+          <a href="https://angular.io/"><img src="assets/angular.png" height="32px" />&nbsp;Angular</a>
           Structural framework for dynamic web apps +
           <a href="https://datatables.net/"><img src="https://datatables.net/media/images/nav-dt.png" />&nbsp;DataTables</a>
           jQuery plug-in for complex HTML tables