Fix checkApiServletClientId placeholder in production config sample

It should not be multiline, as registrar client ids are single short-ish identifiers with no spaces allowed. There's no way for them to span multiple lines.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162389442

java/google/registry/config/files/nomulus-config-production-sample.yaml

@@ -28,9 +28,7 @@ registryPolicy:
   tmchCaMode: PRODUCTION
   tmchCrlUrl: http://crl.icann.org/tmch.crl
   tmchMarksDbUrl: https://ry.marksdb.org
-  checkApiServletClientId: |
-    multi-line
-    placeholder
+  checkApiServletClientId: placeholder
   whoisDisclaimer: |
     multi-line
     placeholder