docs: Remove expose(app) from metrics-diff-port-uvicorn example

[gpkc]

According to the README for the example:

Minimal example that shows usage of Prometheus client library and Prometheus FastAPI Instrumentator with FastAPI and Uvicorn where the /metrics endpoint is exposed on another port and not on the FastAPI app itself.

This suggests that the metrics should NOT be exposed in the app itself for this example. However, calling expose(app) will do exactly that. So we can just remove that call.