fix data_versions toy (#13665)

## Summary & Motivation

## How I Tested These Changes

python_modules/dagster-test/dagster_test/toys/data_versions.py

@@ -18,7 +18,7 @@ def observable_different_version():
 
 @observable_source_asset
 def observable_same_version():
-    return "5"
+    return DataVersion("5")
 
 
 non_observable_source = SourceAsset("non_observable_source")