YARN-6780. ResourceWeights.toString() cleanup (Contributed by weiyuan via Daniel Templeton)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/resource/ResourceWeights.java

@@ -54,8 +54,9 @@ public class ResourceWeights {
     return weights[resourceType.ordinal()];
   }
   
+  @Override
   public String toString() {
-    StringBuffer sb = new StringBuffer();
+    StringBuilder sb = new StringBuilder();
     sb.append("<");
     for (int i = 0; i < ResourceType.values().length; i++) {
       if (i != 0) {