fix: remove extra () from arrow function call in svg/index.js

Bug fix from PR andredumas#220 review comments by hkmoon

Author: gruppler

src/svg/index.js

@@ -4,6 +4,6 @@ import { util } from '../util';
 
 export const svg = function(d3) {
   return {
-    arrow: _arrow(util().functor)()
+    arrow: _arrow(util().functor)
   };
 };