nixos/doc: Fix required disk size for installing in a VirtualBox guest

The manual says that you need at least 8 GB of disk to install in Virtualbox, but the installer returns an error and says that you need at least 10 GB.

Changed files
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nixos
doc
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nixos/doc/manual/installation/installing-virtualbox-guest.section.md
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1. Base Memory Size: 768 MB or higher.
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1. New Hard Disk of 8 GB or higher.
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1. New Hard Disk of 10 GB or higher.
1. Mount the CD-ROM with the NixOS ISO (by clicking on CD/DVD-ROM)