fix: @ in autocomplete textarea template

Author: szuperaz

projects/stream-chat-angular/src/lib/message-input/autocomplete-textarea/autocomplete-textarea.component.html

@@ -71,6 +71,6 @@
     <span data-testclass="username" class="str-chat__user-item--name">{{
       item.autocompleteLabel
     }}</span>
-    <div *ngIf="themeVersion === '2'" class="str-chat__user-item-at">@</div>
+    <div *ngIf="themeVersion === '2'" class="str-chat__user-item-at">&#64;</div>
   </div>
 </ng-template>