DOC: update CONTRIBUTING.md

Author: gorangalinec

CONTRIBUTING.md

@@ -39,7 +39,7 @@ Data Plane API is generated using [go-swagger](https://github.com/go-swagger/go-
     --tags=LogTarget \
     --tags=Reloads \
     --tags=ACL \
-    --tags=Defaults
+    --tags=Defaults \
     -r ~/dataplaneapi-specification/copyright.txt
 ```