Fix DSN example in Readme

The "public" and "secret" keys were reversed.

See Sentry's doc on [writing a client](https://github.com/getsentry/sentry/blob/master/docs/developer/client/index.rst#parsing-the-dsn) for correct format

```
'{PROTOCOL}://{PUBLIC_KEY}:{SECRET_KEY}@{HOST}/{PATH}{PROJECT_ID}'
```