fix: async-in-sync example should use the portal

[cofin]

The flask async documentation is missing the "portal" call for the async from sync call.

async def create_a_user(session: AsyncSession) -> None: 
    user = User(name="I'm Experimental")
    session.add(user)
    await session.commit() 

@app.route("/test", methods=["POST"])
def test_route() -> tuple[dict[str, str], int]:
    session = extension.get_async_session() 
    _ = extension.portal_provider.portal.call(create_user, session)
    return {"status": "success"}, 200