[docs] Fixed snippet mismatch

The snippet shows now the code from lesson 7 (and lesson 9).

docs/dagster-university/pages/dagster-essentials/lesson-8/creating-a-schedule-with-a-date-based-partition.md

@@ -13,9 +13,11 @@ Now that you’ve partitioned the relevant assets, the schedule can be changed t
 Currently, `trip_update_job` in `jobs/__init__.py` should look like this:
 
 ```python
+trips_by_week = AssetSelection.keys("trips_by_week")
+
 trip_update_job = define_asset_job(
-  name="trip_update_job",
-  selection=AssetSelection.all() - AssetSelection.keys(["trips_by_week"]),
+    name="trip_update_job",
+    selection=AssetSelection.all() - trips_by_week
 )
 ```
 
@@ -39,9 +41,11 @@ The job should now look like this:
 from dagster import define_asset_job, AssetSelection, AssetKey
 from ..partitions import monthly_partition
 
+trips_by_week = AssetSelection.keys("trips_by_week")
+
 trip_update_job = define_asset_job(
   name="trip_update_job",
   partitions_def=monthly_partition, # partitions added here
-  selection=AssetSelection.all() - AssetSelection.keys(["trips_by_week"])
+  selection=AssetSelection.all() - trips_by_week
 )
 ```