Fix incorrect flag in nginx-ingress.md (#245)

Originally had `dry-run=true` as the dry run flag but that doesn't work and makes external-dns fail to start. Changed to `--dry-run`.

docs/tutorials/nginx-ingress.md

@@ -227,7 +227,7 @@ spec:
         - --txt-owner-id=my-identifier
 ```
 
-Use `dry-run=true` if you want to be extra careful on the first run. Note, that you will not see any records created when you are running in dry-run mode. You can, however, inspect the logs and watch what would have been done.
+Use `--dry-run` if you want to be extra careful on the first run. Note, that you will not see any records created when you are running in dry-run mode. You can, however, inspect the logs and watch what would have been done.
 
 ## Deploy a sample application