MATH-1555: Fix documentation 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="erans" type="fix" issue="MATH-1555" due-to="Laurent Galluccio">
+        "Atan2": Documentation issue.
+      </action>
       <action dev="erans" type="add" issue="MATH-1551" due-to="Allen Yu">
         "Percentile": Partial support for weighted data.
       </action>

src/main/java/org/apache/commons/math4/analysis/function/Atan2.java

@@ -28,7 +28,7 @@ import org.apache.commons.math4.util.FastMath;
 public class Atan2 implements BivariateFunction {
     /** {@inheritDoc} */
     @Override
-    public double value(double x, double y) {
-        return FastMath.atan2(x, y);
+    public double value(double y, double x) {
+        return FastMath.atan2(y, x);
     }
 }