Fix accessibility on back link

src/app/shared/components/page-header/page-header.component.html

 <div class="page-header">
   <button class="back-button" tabindex="0" (click)="goToPreviousPage()"
     [ngClass]="{'has-beta-style': pageInfo.hasBetaStyle}">
-    <img class="icon-left-arrow" src="./assets/img/left_arrow.svg" alt="Picto de la flèche retour">
+    <img class="icon-left-arrow" src="./assets/img/left_arrow.svg" alt="">
     <ng-container i18n="@@dataset.detail.back">Back</ng-container>
   </button>