chore: fix codeowner team name

This was using an incorrect name.

Author: mrgrain

packages/cdk-assets/.github/CODEOWNERS

@@ -1,4 +1,4 @@
-/.github/ @cdk-core-team
-/bin/ @cdk-core-team
-/lib/ @cdk-core-team
-/test/ @cdk-core-team
+/.github/ @aws-cdk-core-team
+/bin/ @aws-cdk-core-team
+/lib/ @aws-cdk-core-team
+/test/ @aws-cdk-core-team