[new docs] fix multi_asset (#26240)

have to yield these, does nothing if they are not output from the
function

## How I Tested These Changes

eyes

Author: alangenfeld

Diff for: examples/docs_beta_snippets/docs_beta_snippets/guides/data-assets/data-assets/multi_asset_decorator.py

@@ -3,8 +3,8 @@
 
 @dg.multi_asset(specs=[dg.AssetSpec("asset_one"), dg.AssetSpec("asset_two")])
 def my_multi_asset():
-    dg.MaterializeResult(asset_key="asset_one", metadata={"num_rows": 10})
-    dg.MaterializeResult(asset_key="asset_two", metadata={"num_rows": 24})
+    yield dg.MaterializeResult(asset_key="asset_one", metadata={"num_rows": 10})
+    yield dg.MaterializeResult(asset_key="asset_two", metadata={"num_rows": 24})
 
 
 defs = dg.Definitions(assets=[my_multi_asset])