docs: homogenize region tags (#752)

These samples' region tags slightly differ from identical snippets in different languages. The following is the remedy: 1. Merge this PR to fix GitHub source (keep old tag for now) 2. Fix doc code includes to represent the better region tag 3. Submit a second PR to clean-up redundant region tag wrapping

Author: Strykrol

samples/snippets/src/main/java/com/example/spanner/SpannerSample.java

@@ -1622,6 +1622,7 @@ static void createBackup(
  }
  // [END spanner_create_backup]
 
+ // [START spanner_cancel_backup_create]
  // [START spanner_cancel_create_backup]
  static void cancelCreateBackup(
  DatabaseAdminClient dbAdminClient, DatabaseId databaseId, BackupId backupId) {
@@ -1666,6 +1667,7 @@ static void cancelCreateBackup(
  }
  }
  // [END spanner_cancel_create_backup]
+ // [END spanner_cancel_backup_create]
 
  // [START spanner_list_backup_operations]
  static void listBackupOperations(InstanceAdminClient instanceAdminClient, DatabaseId databaseId) {