docs: Update oracle_tables.sql (#549)

Updated the name of index(removed extra underscore)

## Brief, plain english overview of your changes here


## Fixes
<!--- Which issue # does this fix? --->


## Reminders
- [ ] Added/ran automated tests
- [ ] Update README and/or examples
- [ ] Ran `mvn spotless:apply`

---
cc @kagkarlsson

db-scheduler/src/test/resources/oracle_tables.sql

@@ -14,5 +14,5 @@ create table scheduled_tasks
     PRIMARY KEY (task_name, task_instance)
 );
 
-CREATE INDEX scheduled_tasks__execution_time__idx on scheduled_tasks(execution_time);
-CREATE INDEX scheduled_tasks__last_heartbeat__idx on scheduled_tasks(last_heartbeat);
+CREATE INDEX scheduled_tasks_execution_time_idx on scheduled_tasks(execution_time);
+CREATE INDEX scheduled_tasks_last_heartbeat_idx on scheduled_tasks(last_heartbeat);