[Minor] Fix tiny typo in BlockManager

In BlockManager, there is a word `BlockTranserService` but I think it's typo for `BlockTransferService`.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #4046 from sarutak/fix-tiny-typo and squashes the following commits:

a3e2a2f [Kousuke Saruta] Fixed tiny typo in BlockManager

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -34,10 +34,9 @@ import org.apache.spark.executor._
 import org.apache.spark.io.CompressionCodec
 import org.apache.spark.network._
 import org.apache.spark.network.buffer.{ManagedBuffer, NioManagedBuffer}
-import org.apache.spark.network.netty.{SparkTransportConf, NettyBlockTransferService}
+import org.apache.spark.network.netty.SparkTransportConf
 import org.apache.spark.network.shuffle.ExternalShuffleClient
 import org.apache.spark.network.shuffle.protocol.ExecutorShuffleInfo
-import org.apache.spark.network.util.{ConfigProvider, TransportConf}
 import org.apache.spark.serializer.Serializer
 import org.apache.spark.shuffle.ShuffleManager
 import org.apache.spark.shuffle.hash.HashShuffleManager
@@ -120,7 +119,7 @@ private[spark] class BlockManager(
   private[spark] var shuffleServerId: BlockManagerId = _
 
   // Client to read other executors' shuffle files. This is either an external service, or just the
-  // standard BlockTranserService to directly connect to other Executors.
+  // standard BlockTransferService to directly connect to other Executors.
   private[spark] val shuffleClient = if (externalShuffleServiceEnabled) {
     val transConf = SparkTransportConf.fromSparkConf(conf, numUsableCores)
     new ExternalShuffleClient(transConf, securityManager, securityManager.isAuthenticationEnabled())