PYTHON-1729 Add read preference to transaction retry example

Author: ShaneHarvey

test/test_examples.py

@@ -1010,7 +1010,8 @@ def update_employee_info(session):
 
  with session.start_transaction(
  read_concern=ReadConcern("snapshot"),
- write_concern=WriteConcern(w="majority")):
+ write_concern=WriteConcern(w="majority"),
+ read_preference=ReadPreference.PRIMARY):
  employees_coll.update_one(
  {"employee": 3}, {"$set": {"status": "Inactive"}},
  session=session)