This commit adds instructions to install the required dependencies and capitalizes the word "Cosmic" when needed. One punctuation mistake was also fixed
Per my opinion,
>This will create a system-extension inside `cosmic-sysext`, that you can move (without renaming!) into e.g. `/var/lib/extensions`.
could be worded differently, as 'inside' suggests to move the insides of `cosmic-sysext` into `/var/lib/extensions`, and not the whole folder.
When trying to use `systemd-sysext --refresh` the following error is encountered:
`systemd-sysext: unrecognized option '--refresh'`
This PR fixes that.
