fix: correct RateLimiter.start_under_sentry_supervisor/2 spec

[lukaszsamson]

Found with SpecLint on elixir 1.20.4

lib/sentry/logger_handler/rate_limiter.ex:16: warning: Sentry.LoggerHandler.RateLimiter.start_under_sentry_supervisor/2: inferred return includes :ok, which the spec does not declare
    @spec start_under_sentry_supervisor(
  id(),
  keyword()
) :: Supervisor.on_start_child()
    inferred: (atom(), empty_list() or non_empty_list(term(), term())) -> dynamic(:ok or {:error, not {:already_started, term()}})

The function maps Supervisor.start_child/2 results to :ok (including {:error, {:already_started, _}}) or {:error, reason}, so it never returns {:ok, pid}; the spec now says :ok | {:error, term()}. Spec-only change; no CHANGELOG entry since it is generated at release time.