Doc: Resizing Boot2Docker Volume with VirtualBox

The Boot2Docker VM ships with a 20G VMDK volume, which can quickly fill up if one is using large images/large numbers of images. This is a step-by-step guide for increasing the B2D volume using VirtualBox and GParted.
I understand, however, somehow, this sounds bit odd.

First, I think it should rather be "VM volume" (not "VM's volume").

If we're sticking with ".. VM's volume" (which sounds wrong), going with "your Boot2Docker [..]" (for some reason), or just "Boot2Docker VM's volume" would be better IMO.

Please feel free to ignore my comments if this is, as is, OK with everybody else.

All in all, I'd try to simplify the paragraph:

> When working with large images (both in numbers, and size), `pull`s might start to fail
> with a "No space left on device" error. This is due to Boot2Docker VM volume filling up.
> The solution is to increase the volume size by first cloning, and then resizing using a
> disk partitioning tool, such as the free [GParted](http://gparted.sourceforge.net/download.php/index.php)
> partitions management application.