fix: async transactional

[rafilong]

Expected usecase of AsyncTransactional is as follows, update implementation to await wrapped function.

async def update_data_transaction():
    db = firestore.AsyncClient()
    transaction = db.transaction()
    city_ref = db.collection(u'cities').document(u'SF')

    @firestore.async_transactional
    async def update_in_transaction(transaction, city_ref):
        snapshot = await city_ref.get(transaction=transaction)
        transaction.update(city_ref, {
            u'population': snapshot.get(u'population') + 1
        })

    await update_in_transaction(transaction, city_ref)