centos-image-resize -> linux-rootfs-resize

The image guide links to an external project to build resizeable
virtual machine images. Its name changed from centos-image-resize
to linux-rootfs-resize.

Update the image guide to reflect this change.

Change-Id: I802530237aee91c110f9241497a0a2f0988e0c83
Merge "centos-image-resize -> linux-rootfs-resize"