docs: Update emulator sample to create a topic and publish to it

[michaelpri10]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

If you write sample code, please follow the samples format.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/pubsub/UsePubSubEmulatorExample.java:25, tag pubsub_use_emulator

You are about to delete 1 region tag.

• samples/snippets/src/main/java/pubsub/UsePubSubEmulatorExample.java:40, tag pubsub_use_emulator

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[michaelpri10]

Note that this report is in response to internal code sample feedback (312895180).