Use the correct variable (#6406)

The merge command should be returning the start and end rows for the
merge op.

If a user only defined an endRow arg then the op message would state that
they were merging "-inf" to "+inf".

Author: ddanielr

server/manager/src/main/java/org/apache/accumulo/manager/FateServiceHandler.java

@@ -466,7 +466,7 @@ public void executeFateOperation(TInfo tinfo, TCredentials c, long opid, FateOpe
         }
 
         String startRowStr = StringUtils.defaultIfBlank(startRow.toString(), "-inf");
-        String endRowStr = StringUtils.defaultIfBlank(startRow.toString(), "+inf");
+        String endRowStr = StringUtils.defaultIfBlank(endRow.toString(), "+inf");
 
         Manager.log.debug("Creating merge op: {} from startRow: {} to endRow: {}", tableId,
             startRowStr, endRowStr);