docs(compiler): fix variable scope in drag sample

Note that without this fix, if you add a second draggable element, the
two instances clobber each other since there is only one set of
startx/starty/x/y variables.
Here is an example: http://plnkr.co/edit/aGrLXcIo2SuaePuAdfmQ?p=preview.
On the surface it looks like it would be fine because you only have one
mouse but in practice the start position jumps when you start dragging.
Here it is fixed: http://plnkr.co/edit/VuvPasuumtCeiVRisYKQ?p=preview

Author: keir

docs/content/guide/compiler.ngdoc

@@ -70,8 +70,8 @@ Here is a directive which makes any element draggable. Notice the `draggable` at
   <file name="script.js">
     angular.module('drag', []).
       directive('draggable', function($document) {
-        var startX=0, startY=0, x = 0, y = 0;
         return function(scope, element, attr) {
+          var startX = 0, startY = 0, x = 0, y = 0;
           element.css({
            position: 'relative',
            border: '1px solid red',