Change reccomendation of applying middleware

[fraser-langton]

change to:

from fastapi import FastAPI

from asgi_correlation_id import CorrelationIdMiddleware

fastapi_app = FastAPI()
app = CorrelationIdMiddleware(fastapi_app)

I think this aligns better with expected behaviour (imo not returning correlation id on 500s is no expected).

I see you have a section to alleviate this behaviour but I think it should be default behaviour - also it feels a little cleaner just applying it once and not having to replicate the logic in a exc handler