MATH-1285: replaced inappropriate reference to MathWorld by reference to

Wikipedia's article about Zipf's law, thanks to  Pim van der Hoorn for
reporting this issue

src/changes/changes.xml

@@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
     </release>
 
     <release version="4.0" date="XXXX-XX-XX" description="">
+      <action dev="oertl" type="fix" issue="MATH-1285" due-to="Pim van der Hoorn "> <!-- backported to 3.6 -->
+        Updated reference in ZipfDistribution's javadoc.
+      </action>
       <action dev="tn" type="fix" issue="MATH-1283" due-to="Jean Noel Delavalade"> <!-- backported to 3.6 -->
         Fixed "Gamma#gamma(double)" for negative values smaller than -20.
       </action>

src/main/java/org/apache/commons/math4/distribution/ZipfDistribution.java

@@ -26,7 +26,7 @@ import org.apache.commons.math4.util.FastMath;
 /**
  * Implementation of the Zipf distribution.
  *
- * @see <a href="http://mathworld.wolfram.com/ZipfDistribution.html">Zipf distribution (MathWorld)</a>
+ * @see <a href="https://en.wikipedia.org/wiki/Zipf's_law">Zipf's law (Wikipedia)</a>
  */
 public class ZipfDistribution extends AbstractIntegerDistribution {
     /** Serializable version identifier. */