Use actual name of flag in error message.

RELNOTES: n/a
PiperOrigin-RevId: 335055802

src/main/java/com/google/devtools/build/lib/worker/WorkerSpawnRunner.java

@@ -202,7 +202,7 @@ private SpawnResult actuallyExec(Spawn spawn, SpawnExecutionContext context)
       if (protocolFormat == WorkerProtocolFormat.JSON) {
         throw new IOException(
             "Persistent worker protocol format must be set to proto unless"
-                + " --experimentalJsonWorkerProtocol is used");
+                + " --experimental_allow_json_worker_protocol is used");
       }
     }