[MINOR] improvement: Fix logger classname in RssMergeManager (#2435)

### What changes were proposed in this pull request?

Fixing logger classname.

### Why are the changes needed?

Logger classname was MergeManager in RssMergeManager class.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

UT

Author: asalamon74

client-tez/src/main/java/org/apache/tez/runtime/library/common/shuffle/orderedgrouped/RssMergeManager.java

@@ -52,7 +52,7 @@
 
 public class RssMergeManager extends MergeManager {
 
-  private static final Logger LOG = LoggerFactory.getLogger(MergeManager.class);
+  private static final Logger LOG = LoggerFactory.getLogger(RssMergeManager.class);
 
   private Configuration conf;
   private InputContext inputContext;