chore: update CODEOWNERS (#259)

Author: stephaniewang526

.github/CODEOWNERS

@@ -4,6 +4,9 @@
 # For syntax help see:
 # https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners#codeowners-syntax
 
+# The @googleapis/api-bigquery is the default owner for changes in this repo
+*               @googleapis/api-bigquery
 
-/samples/   @shollyman @googleapis/python-samples-owners
+# The python-samples-reviewers team is the default owner for samples changes
+/samples/   @googleapis/python-samples-owners