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