fix: Update index.md

[seunghkong]

fix to the following type hint error:

Argument of type "(request: Request, exc: RateLimitExceeded) -> Response" cannot be assigned to parameter "handler" of type "ExceptionHandler" in function "add_exception_handler"
  Type "(request: Request, exc: RateLimitExceeded) -> Response" is not assignable to type "ExceptionHandler"
    Type "(request: Request, exc: RateLimitExceeded) -> Response" is not assignable to type "HTTPExceptionHandler"
      Parameter 2: type "Exception" is incompatible with type "RateLimitExceeded"
        "Exception" is not assignable to "RateLimitExceeded"

practically the same code as

app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)