samples: Update asset lib to v1 for ListAssets sample code (#204)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: peter-zheng-g

asset/snippets/snippets/quickstart_listassets.py

@@ -20,7 +20,7 @@
 
 def list_assets(project_id, asset_types, page_size):
     # [START asset_quickstart_list_assets]
-    from google.cloud import asset_v1p5beta1
+    from google.cloud import asset_v1
 
     # TODO project_id = 'Your Google Cloud Project ID'
     # TODO asset_types = 'Your asset type list, e.g.,
@@ -29,10 +29,10 @@ def list_assets(project_id, asset_types, page_size):
     # 1000 (both inclusively)'
 
     project_resource = "projects/{}".format(project_id)
-    content_type = asset_v1p5beta1.ContentType.RESOURCE
-    client = asset_v1p5beta1.AssetServiceClient()
+    content_type = asset_v1.ContentType.RESOURCE
+    client = asset_v1.AssetServiceClient()
 
-    # Call ListAssets v1p5beta1 to list assets.
+    # Call ListAssets v1 to list assets.
     response = client.list_assets(
         request={
             "parent": project_resource,