fixed typo

lib/network/shapes.js

@@ -206,7 +206,7 @@ if (typeof CanvasRenderingContext2D !== 'undefined') {
 
 
   /**
-   * Draw an arrow an the end of an line with the given angle.
+   * Draw an arrow at the end of a line with the given angle.
    */
   CanvasRenderingContext2D.prototype.arrowEndpoint = function (x, y, angle, length) {
     // tail