Add larkee as codeowner of spanner (#3951)

Author: mmmarklu

.github/CODEOWNERS

@@ -51,7 +51,7 @@
 /pubsub/                        @anguillanneuf @hongalex @GoogleCloudPlatform/python-samples-owners
 /run/                           @averikitsch @GoogleCloudPlatform/python-samples-owners
 /scheduler/                     @averikitsch @GoogleCloudPlatform/python-samples-owners
-/spanner/                       @mmmarklu @GoogleCloudPlatform/python-samples-owners
+/spanner/                       @larkee @GoogleCloudPlatform/python-samples-owners
 /speech/                        @telpirion @sirtorry @GoogleCloudPlatform/python-samples-owners
 /storage/                       @GoogleCloudPlatform/storage-dpe @GoogleCloudPlatform/python-samples-owners
 /tables/automl/                 @telpirion @sirtorry @GoogleCloudPlatform/python-samples-owners