Fix typo in copy() code

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@902934 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/math/stat/descriptive/moment/FirstMoment.java

@@ -154,6 +154,6 @@ public class FirstMoment extends AbstractStorelessUnivariateStatistic
         dest.n = source.n;
         dest.m1 = source.m1;
         dest.dev = source.dev;
-        dest.nDev = dest.nDev;
+        dest.nDev = source.nDev;
     }
 }