Fix comment on WorkflowExecutionCollection#with_status and #count

A correct value of `closeStatusFilter.status` is `continue_as_new` but `continue`.

```ruby
swf=AWS::SimpleWorkflow.new(region: 'ap-northeast-1')
domain = swf.domains['TestDomain']
domain.workflow_executions.with_status(:continue).count
# => AWS::SimpleWorkflow::Errors::ValidationException 1 validation error detected: Value 'CONTINUE' at 'closeStatusFilter.status' failed to satisfy constraint: Member must satisfy enum value set: [TERMINATED, TIMED_OUT, COMPLETED, CONTINUED_AS_NEW, CANCELED, FAILED]
```