HDFS-15758. Fix typos in MutableMetric. Contributed by Haibin Huang.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/lib/MutableMetric.java

@@ -56,7 +56,7 @@ public abstract class MutableMetric {
   protected void clearChanged() { changed = false; }
 
   /**
-   * @return  true if metric is changed since last snapshot/snapshot
+   * @return  true if metric is changed since last snapshot
    */
   public boolean changed() { return changed; }
 }