FlowableMultiInstanceActivityEventImpl#isSequential() typo issue

The method `isSequential` return false regardless of the value set.

```java
public class FlowableMultiInstanceActivityEventImpl extends FlowableActivityEventImpl implements FlowableMultiInstanceActivityEvent{
protected boolean sequential;

public FlowableMultiInstanceActivityEventImpl(FlowableEngineEventType type) {
super(type);
}

public void setSequential(boolean sequential) {
this.sequential = sequential;
}

@Override
public boolean isSequential() {
return false;
}

}
```

Locate FlowableMultiInstanceActivityEventImpl and inspect the setSequential(boolean) and isSequential() methods. Verify the event reports the value assigned through the setter, then run the relevant existing Java test suite or add coverage for both sequential values if the project has no matching test.

Written by the indexing model from the issue text.