Typo in project string in upgrading guide

https://github.com/googleapis/python-pubsub/blob/792c56582ed1b8a3dab4b5b1bc751131be56de00/UPGRADING.md?plain=1#L152

This should be:

```
project_path = f"projects/{PROJECT_ID}"
```

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Here is a line that seems correct:
https://github.com/googleapis/python-pubsub/blob/792c56582ed1b8a3dab4b5b1bc751131be56de00/UPGRADING.md?plain=1#L57