fix declared function name

Author: ctsims

src/main/java/org/javarosa/xpath/expr/XPathPolygonDistanceFunc.java

@@ -12,7 +12,7 @@
 import java.util.List;
 
 public class XPathPolygonDistanceFunc extends XPathFuncExpr{
-    public static final String NAME = "boundaryDistance";
+    public static final String NAME = "polygon-distance";
     private static final int EXPECTED_ARG_COUNT = 2;
 
     public XPathPolygonDistanceFunc() {
@@ -59,4 +59,4 @@ public static Double boundaryDistance(Object from, Object to) {
                     "numeric values only, but received arguments: " + unpackedFrom + " and " + unpackedTo);
         }
     }
-}
+}