Add method signature

Author: kgryte

lib/node_modules/@stdlib/plot/components/svg/rug/README.md

@@ -142,7 +142,7 @@ var data = node.data;
 
 <a name="property-isdefined"></a>
 
-#### Rug.prototype.isDefined
+#### Rug.prototype.isDefined( d, i )
 
 **Writable** property whose value is a predicate `function` which indicates whether a datum is defined and thus determines how missing values are encoded. When invoked, the function is provided two arguments: