[doc] Improve s3 operator example by adding task upload_keys

[zhongjiajie]

for now, our s3 bucket example does without declaring task upload_keys but we set dependence with it. I think we should better add upload_keys declare to the example for more sence.

[ferruzzi]

I can see what you are saying, yeah. I'm working on planning a bit of a project to get a lot of the AWS examples and docs up to par, so there should be a lot of improvements in that area in the near future. I'll keep cases like this in mind.

[zhongjiajie]

@ferruzzi So should we merge or should we just close this issue?

[ferruzzi]

@zhongjiajie I say go ahead, it's non-breaking either way and I think we're going to focus on the ones that don't have any documentation or sample DAGs first so I don't see a reason to block this just because we might update it later. I'm not a committer but you have my non-binding approval :P

[zhongjiajie]

Thanks~