Update dataset definition to match Getting Started

* Use `create_dataset()` instead of `Dataset`
* Let the reader add the age column themselves

analysis/dataset_definition.py

@@ -1,14 +1,14 @@
-from ehrql import Dataset
+from ehrql import create_dataset
 from ehrql.tables.beta.tpp import patients, practice_registrations
 
-dataset = Dataset()
+dataset = create_dataset()
 
 index_date = "2020-03-31"
 
 has_registration = practice_registrations.for_patient_on(
     index_date
 ).exists_for_patient()
 
-dataset.age = patients.age_on(index_date)
+dataset.define_population(has_registration)
 
-dataset.define_population(has_registration & (dataset.age > 17))
+dataset.sex = patients.sex