docs: Capital letter at start and period at end of sentence
Fix two nits in the Debian install guide.
>
> - This guide assumes you have
> [already installed the Kata Containers packages](../debian-installation-guide.md).
-> - this guide allows for installation with `systemd` or `sysVinit` init systems
+> - This guide allows for installation with `systemd` or `sysVinit` init systems.
1. Install Docker with the following commands:
