message: fix(docs-infra): amend notification close button aria-label
  move the aria-label used inside the close-button to the button itself
  (since otherwise the button's aria-label overrides the childs)
  PR Close
diff: <ng-content></ng-content>
  </span>
  -<button mat-icon-button class="close-button" aria-label="Close" (click)="dismiss()">
  - <mat-icon svgIcon="close" aria-label="Dismiss notification"></mat-icon>
  +<button mat-icon-button class="close-button" aria-label="Dismiss notification" (click)="dismiss()">
  + <mat-icon svgIcon="close"></mat-icon>
  </button>
  