Fix docstring example: session.begin_transaction() needs to be awaited (#807)

Co-authored-by: Robsdedude <rouven.bauer@neo4j.com>

Author: 112358fn

neo4j/_async/work/transaction.py

@@ -256,7 +256,7 @@ class AsyncTransaction(AsyncTransactionBase):
     managers (:py:const:`async with` block) where the transaction is committed
     or rolled back on based on whether an exception is raised::
 
-        async with session.begin_transaction() as tx:
+        async with await session.begin_transaction() as tx:
             ...
 
     """