refactor: use proper bootstrap class

Author: ghiscoding

src/app/examples/grid-tree-data-hierarchical.component.html

@@ -28,7 +28,7 @@ <h2 [innerHTML]="title"></h2>
       <div class="input-group">
         <input type="text" class="form-control search-string" data-test="search-string"
                [(ngModel)]="searchString" (ngModelChange)="searchStringChanged()">
-        <div class="input-group-btn">
+        <div class="input-group-append">
           <button class="btn btn-outline-secondary" data-test="clear-search-string" (click)="clearSearch()">
             <span class="icon mdi mdi-close-thick"></span>
           </button>