fix javadocs

Author: igorbernstein2

google-cloud-clients/google-cloud-bigtable-admin/src/main/java/com/google/cloud/bigtable/admin/v2/BigtableInstanceAdminClient.java

@@ -296,7 +296,6 @@ public List<Instance> listInstances() {
    *
    * <pre>{@code
    * ApiFutures.addCallback(instancesFuture, new ApiFutureCallback<List<Instance>>() {
-   *   @Override
    *   public void onFailure(Throwable t) {
    *     if (t instanceof PartialListInstancesException) {
    *       PartialListInstancesException partialError = (PartialListInstancesException)t;
@@ -305,7 +304,6 @@ public List<Instance> listInstances() {
    *     }
    *   }
    *
-   *   @Override
    *   public void onSuccess(List<Instance> result) {
    *     System.out.println("Found a complete set of instances: " + result);
    *   }