Update pubsub code examples

[Fokko]

Hi all,

There are some changes in the recent API updates. I've updated the code examples to make it easier for others to conform to the changes. The current code examples are compatible with pubsub 0.40.0-beta.

Cheers, Fokko

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[Fokko]

I signed it!

[googlebot]

CLAs look good, thanks!

[pongad]

Thank you very much for this.

Google Java style guide asks for 2-space indentation though. Could you please make this change. Other than that, this LGTM.

[Fokko]

Thanks @pongad for reviewing. I've revised the commit with two spaces. Cheers