fixed the since message to show the getSparsity has been introduced in 2.2

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1073263 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/math/linear/OpenMapRealVector.java

@@ -839,6 +839,7 @@ public class OpenMapRealVector extends AbstractRealVector
     /**
      *
      * @return the percentage of none zero elements as a decimal percent.
+     * @since 2.2
      */
     public double getSparsity() {
         return (double)entries.size()/(double)getDimension();