Fix minimum root partition size regex

I'm trying to install arch using the anarchy installer and I'm hitting this issue where I'm unable to select my 1.3T partition as the root partition. I'm getting:

Error: partition too small to be root partition. *Root partition must be at least 4 GiB in size.

The previous regex would not match the entire input as seen here:

There might be a better way to write this regex but I guess these changes should work.

Fixes #141