Fix the cloud-initramfs-growroot package name.

Docs previously listed the "cloud-initramfs-tools" package as part of the simplest way to enable automatic root filesystem resizing on boot. That's just a source package, which you can't actually install. cloud-initramfs-growroot contains the actual hooks for the growroot action.

Change-Id: Iccedeffd92c90772c504affafac6c65a302240b2
Closes-Bug: #1331179