riders list: Rename the PDF creation button to PDF

With the additional forms to print listed here, "Fahrerliste" becomes a bit confusing.

htdocs/admin/event/list/view.html

@@ -196,7 +196,7 @@ <h1><a href="" ng-click="settings($event)">{{event.title}}</a></h1>
 	    </td>
 	  </tr>
 	  <tr><th></th>
-	    <td><button type="submit" ng-click="create_pdf($event)">Fahrerliste</button></td>
+	    <td><button type="submit" ng-click="create_pdf($event)">PDF</button></td>
 	  </tr>
 	  <tr ng-repeat="form in config.pdf_forms[event.type]" ng-if="!groups"><th></th>
 	    <td>