[MINOR] Add missing interpolation in NettyRPCEnv

```
Exception in thread "main" org.apache.spark.rpc.RpcTimeoutException:
Cannot receive any reply in ${timeout.duration}. This timeout is controlled by spark.rpc.askTimeout
	at org.apache.spark.rpc.RpcTimeout.org$apache$spark$rpc$RpcTimeout$$createRpcTimeoutException(RpcTimeout.scala:48)
	at org.apache.spark.rpc.RpcTimeout$$anonfun$addMessageIfTimeout$1.applyOrElse(RpcTimeout.scala:63)
	at org.apache.spark.rpc.RpcTimeout$$anonfun$addMessageIfTimeout$1.applyOrElse(RpcTimeout.scala:59)
	at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
```

Author: Andrew Or <andrew@databricks.com>

Closes #10334 from andrewor14/rpc-typo.